Add tests for threaded resource loading

[myaaaaaaaaa]

For #73647

I've only been able to test the non-threaded sections, so if this fails (which it probably will), it would be due to incorrect usage of ResourceLoader's threading functions.

@RandomShaper Present for you 🙂

[RandomShaper]

Playing with this plus my #71280 at https://github.com/RandomShaper/godot/tree/fix_deadlock_windows_with_tests.

[RandomShaper]

I've done a mashup of threading fixing and testing PRs. Thread sanitizer seems OK now!
https://github.com/RandomShaper/godot/commits/fix_deadlock_windows_with_tests

After 4.0 is released, I guess all these PRs can be merged. They will be good for 4.0.1 and also set a good basis for the threading safety overhaul that will happen on 4.1.

[mrTag]

I wrote a unit test to help @RandomShaper with the threaded (or dreaded? 😄) resource loading fixes #74405 (comment)
Have a look at the code here: https://gist.github.com/mrTag/117ba6737ab9cee789cb4835dc702fe2 maybe it would make sense to include it in this PR? Or maybe your "[Resource] Thread safety test" does something similar? It freezes on my system and I didn't quite get what it wanted to test.