Resolve "Add missing karma tests to the the MR Diff components"

What does this MR do?

Adds test for MR widget components

These tests need to be added:

• spec/javascripts/diffs/components/app_spec.js

• spec/javascripts/diffs/components/changed_files_dropdown_spec.js - component was deleted

• spec/javascripts/diffs/components/compare_versions_dropdown_spec.js

• spec/javascripts/diffs/components/compare_versions_spec.js

• spec/javascripts/diffs/components/diff_content_spec.js

• spec/javascripts/diffs/components/edit_button_spec.js

• spec/javascripts/diffs/components/hidden_files_warning_spec.js

• spec/javascripts/diffs/components/no_changes_spec.js

• spec/javascripts/diffs/components/diff_file_header_spec.js Missing tests for file actions.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #55505 (closed)