Add support for specific simulator IDs for iOS tests

[keith]

This allows users to pass configurable simulator UDIDs to run tests
with. This allows them to run test jobs on the same simulator without
the overhead of creating one each time.

Another benefit of this is if you want to setup simulators ahead of time for example for UI tests where you may need to set some simulator defaults to not show iOS' onboarding flows

[keith]

Example usage:

string_flag(
    name = "ios_simulator_id",
    build_setting_default = "",
    visibility = ["//visibility:public"],
)

ios_test_runner(
    name = "custom_test_runner",
    simulator_id = "//:ios_simulator_id",
    visibility = ["//visibility:public"],
)

bazel test //... --//:ios_simulator_id="21461F1A-68DD-43E0-A795-D0329CB1E217"

I would have put this string_flag in rules_apple but it seems that you can not use it from external repos, so --@build_bazel_rules_apple//apple:ios_simulator_id=... doesn't work

[thomasvl]

Can you file a bug against bazel for rules sets not being able to define these and have them work? We'll get it raised internally.

[keith]

I filed bazelbuild/bazel#10152 about this. I think in the meantime this would probably still be a fine way to handle this, but I guess it depends on how you feel about the public API changing if this starts working because I initial made simulator_id a private attr referencing the flag that was in rules_apple

[khandpur]

Can you use test arguments for this? we've added support for these in our internal test runner:
“--destination=platform=ios_device,id={UUID}” targets on a local iOS real device.
“--destination=platform=ios_simulator,id={UUID}” targets on an existing iOS simulator.

and invoke it with blaze as such:
blaze test <target> --test_arg=--destination=id={UUID},platform=ios_simulator or you can also pass it in args of the ios_{unit,ui}_test targets in the BUILD file

[keith]

I've submitted the --test_arg option here #653

[keith]

#653 is the better approach