Convert axisAlignedCollision tasts to Catch2 and improve docs

[JosiahWI]

This is just cleanup. The content of the unit tests should not be changed, although one redundant test was removed. It looked like the author copy-pasted a test, intending to change the test parameters, but forgot to change them. Since I don't know what the author intended to change, I took it back out. I have not converted the collisionMoveSimple tests that were recently added, and I do not have time to in the near future.

I have also reworded the docstring of the function under test to clarify that dtime is an inout parameter. The original wording does not seem to mention its use as an input to the function. This information came from @appgurueu, and I observed it to be correct based on the unit test behavior; I have not read the code of the function under test.

To do

Ready for Review.

How to test

luanti --run-unittests --test-module "[collision]"

[kno10]

#15408 adds additional collision unit tests.

[JosiahWI]

@kno10 That looks great, thank you! If that lands before this one, I can rebase this over some weekend thereafter.