C++ refactoring: UnionArray argsort and sort and enable tests

[ianna]

No description provided.

[codecov]

Codecov Report

Merging #1141 (b1bb82d) into main (0b0fb3f) will increase coverage by 0.90%.
The diff coverage is 66.21%.

Impacted Files	Coverage Δ
src/awkward/_v2/behaviors/string.py	100.00% <ø> (ø)
src/awkward/_v2/forms/bitmaskedform.py	67.94% <ø> (ø)
src/awkward/_v2/forms/bytemaskedform.py	69.01% <ø> (ø)
src/awkward/_v2/forms/indexedform.py	72.00% <ø> (+8.84%)	⬆️
src/awkward/_v2/forms/unmaskedform.py	69.49% <ø> (ø)
src/awkward/_v2/highlevel.py	41.37% <ø> (+6.26%)	⬆️
.../awkward/_v2/operations/convert/ak_from_buffers.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_cupy.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_iter.py	80.00% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_from_jax.py	80.00% <0.00%> (ø)
... and 126 more

[jpivarski]

This is probably right: I wrote the first comment thinking it was for a different function—if I was wrong about that and there's nothing wrong with the code I pointed out, then this PR is 100% ok and if ready to be merged.

[ianna]

@jpivarski - thanks for checking. This PR is a short one, I'm done with it.