Add missing karma tests to the the MR Diff components

Spinning this off from: https://gitlab.com/gitlab-org/gitlab-ce/issues/48034

These tests need to be added:

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

• spec\javascripts\diffs\components\changed_files_dropdown_spec.js

• 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.