Add a CI job to enable/disable feature flags in GDK tests
When an MR includes a feature flag definition file, we add a job to package-and-test
pipelines that toggles the feature flag (sets it to the opposite of the default value) so that each test is run once with the flag set to the default value, and once with the flag toggled.
We should have the same coverage in the GDK pipeline - add a job, allowed to fail, that sets the feature flag to the opposite of its default value.
For a bonus, we could add a discussion that notifies the MR author if tests fail with the feature flag toggle, to let them know e2e tests need to be updated. But before doing this we should be wary of overloading MRs with bot comments, and try to make sure it's clear, and clearly actionable.
Note: gitlab-org/gitlab!138877 (merged) sets QA_FEATURE_FLAGS=""
as a workaround because the feature flag toggling for package-and-test
was inadvertently toggling the feature flag in the GDK jobs, causing them to fail. See gitlab-org/gitlab!137890 (comment 1681217315)