Cleanup: catch2 now allows multiple threads

### Describe the bug

I previously filed Issue #4079 which Anurag fixed in PR #4155 back when catch2 changed to not allow multiple threads.
At that time, I filed https://github.com/catchorg/Catch2/issues/2935 against catch2 due to this regression.
That issue was just fixed via an opt-in macro.

So we can finally clean up our code again by revisting/reverting 4155 and cleaning up any subsequently added workarounds.

### OS information

_No response_

### Steps taken to reproduce bug

Code inspection.

### Expected behavior

Go back to cleaner implementation approach.

### Actual outcome

Currently have workarounds in place.

### Additional details

_No response_