Raise an error for flag stub typos

What does this MR do and why?

Ensure that test fail if the stubed feature flag name is not defined.

Work that supported this. Removed the offending stubs: #378182

Steps to reproduce

1. Find an existing spec with stub_feature_flags
2. Change the feature flag name to something invalid
3. Run that spec
4. Watch it fail

  2) EE::Gitlab::Checks::PushRuleCheck#validate! :parallel_push_checks feature is disabled behaves like push checks raises an error on failure
     Failure/Error: RSpec::Expectations.fail_with(e.message)
       Invalid flag name: mola_mola

MR acceptance checklist

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