We cannot modify the list of build targets as we're iterating over it. Instead of removing non-test/non-executable elements, instead we add executable test targets to a separate testTargets list and use that.

Noticed while working on: flutter/flutter#147071

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_gnDesc`: returns the JSON output of `gn desc buildDir target`
* `_gnOutputs`:  returns the output files listed by `gn outputs buildDir target`

Noticed while working on: flutter/flutter#147071

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_runGnDesc`: returns the JSON output of `gn desc buildDir target`
* `_runGnOutputs`: returns the output files listed by `gn outputs
buildDir target`

Noticed while working on:
flutter/flutter#147071

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat