Switch to mocha repoter for karma specs

What does this MR do?

This makes mocha the default reporter in karma specs, as there is much more detail provided about which tests that fail and why:

  ZenMode
    enabling dropzone
      ✔ should not call dropzone if element is not dropzone valid
      ✔ should call dropzone if element is dropzone valid
    on enter
      ✔ pauses Mousetrap
      ✔ removes textarea styling
    in use
      ✔ exits on Escape
    on exit
      ✔ unpauses Mousetrap
      ✔ restores the scroll position
  test errors
    ✔ has no unhandled Promise rejections
    ✔ has no Vue warnings
    ✔ has no Vue error
    ✔ restores axios adapter after mocking

Finished in 1 min 23.069 secs / 1 min 21.649 secs @ 13:14:49 GMT+0000 (Coordinated Universal Time)

SUMMARY:
✔ 5489 tests completed
✖ 1 test failed

FAILED TESTS:
  DiffsStoreActions
    setInlineDiffViewType
      ✖ should set diff view type to inline and also set the cookie properly
        Chrome
      Uncaught TypeError: Cannot read property 'reduce' of undefined thrown

What are the relevant issue numbers?

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 #58716 (closed)