Touching a feature flag file should run associated RSpec tests

In gitlab-org/quality/engineering-productivity/master-broken-incidents#1401 (closed), we discovered that touching a feature flag file resulted in a broken master since it didn't trigger unit, integration, nor background migration tests there.

Ideally, when a feature flag is touched, we should find all the files where it's called (e.g. git grep --files-with-matches s3_multithreaded_uploads . | grep '\.rb$'), and add them to the list of "changed files".