Include axe checks in `spec/features/issues/discussion_lock_spec.rb` feature test

Accessibility tests not detected. Refer to https://docs.gitlab.com/ee/development/fe_guide/accessibility.html#separate-file-for-extensive-test-suites on how to include them.

Note: Locking a discussion was moved from an issue's sidebar to the dropdown, but is not reflected in the feature tests yet. Please add first corresponding feature tests for when the feature flag is turned on. Then include accessibility checks for both enable and disabled feature flag.