Add support for filtering tests based on "features" in runtests.pl

[aquilamacedo]

Hi, this is comming for a discussion on Debian, from
https://salsa.debian.org/debian/curl/-/merge_requests/39

We’ve been planning to run tests that rely on the "Debug" feature
because some tests developed to validate CVE fixes (such as tests/data/test780,
tests/data/test781, etc.) require a Debug build in order to run.

However, when running the tests, we need to execute only those that have the
Debug feature set. Initially, we considered using keyword-based filtering
(runtests.pl [keyword]), but I noticed that it doesn’t work because the
parser only checks values inside the <keywords> field, ignoring <features>.

We want to scope the test run to Debug-tests-only because we will do an extra
build of curl (Debug build) just for them, whereas our regular build will run
the non-Debug tests. We want to save time by not runing the non-debug tests
twice.

So, I'm starting this thread to understand the best approach to handle this
scenario.

We see three possible alternatives:

1. Add a new parameter (--feature=) to runtests.pl to allow selecting tests
   based on <features>. Example: --feature=debug.

2. Modify the current keyword filtering behavior ([keyword]) to also consider
   <features>.

3. Set "Debug" as a test keyword so it works with the existing option.
   However, this approach would require modifying all tests that have it as a
   feature and duplicating it to the field.

Any other ideas?

I'm willing to implement the solution and open a PR, depending on which
alternative is considered the best

[bagder]

I think I like option 2 the most. If it turns out to match too many tests too easily, maybe we could have prefix like feat: on the words for feature matches. Like ./runtests.pl feat:debug.