sets minor improvement

[ringabout]

No description provided.

[timotheecour]

• carefully reviewed, merging.
  (was: assert => doAssert + a move)

• note:
  these maybe(didn't check) correspond to builtin sets:
  tests/sets//t2669.nim
  tests/sets//tsets_various.nim
  tests/sets//tsets.nim

• right now we have:

tests/collections/tcollections.nim
tests/collections/tcollections_to_string.nim
tests/collections/thashsets.nim
tests/collections/tseq.nim
tests/collections/ttables.nim
tests/collections/ttablesthreads.nim

given that the stdlib name is sets.nim (different from builtin sets), it'd be more logical to have
the test for sets.nim be under
tests/collections/tsets.nim
instead of
tests/collections/thashsets.nim

note that nim now allows multiple module with same name in same project, so
having these:
tests/collections/tsets.nim
tests/sets//tsets.nim

shouldn't cause conflicts even if merged via megatest.

[ringabout]

If you see the commits before, you could find the CI failures due to name conflicts(tsets)

[timotheecour]

I can't find the logs? https://github.com/nim-lang/Nim/runs/1435593398
curious why it'd fail

but how about this: we could rename tests/sets//tsets.nim to tests/sets//tbuiltin_sets.nim and tests/collections/thashsets.nim to tests/collections/tsets.nim

[ringabout]

tests/collections/thashsets.nim exists before, I'm just moving tests to them.

CI failures:

megatest.nim(910, 8) Error: redefinition of 'tsets'; previous declaration here: stdlib/tsets.nim(1, 2)

https://builds.sr.ht/~araq/job/348186