Skip Feature.enabled? override in test environment

What does this MR do and why?

Skip Feature.enabled? override in test environment

The enable_introspection_feature_flags rake task overrides Feature.enabled? with a singleton method, but this breaks the test environment where StubbedFeature is prepended and calls super. Skipping the override in tests prevents polluting the test environment and causing flaky tests.

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Before	After

How to set up and validate locally

Before this MR, the following execution failed, because Feature.enabled? is still overriden and would return true:

bundle exec rspec spec/tasks/gitlab/graphql_introspection_rake_spec.rb spec/models/work_items/types_filter_spec.rb

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