fix some ruff warnings in tests files

[even-even]

Hello!

I used https://github.com/astral-sh/ruff in mutmut and I want to fix some warnings)))

In tests files:

1. Delete twice check ('import foo', []),
2. sorting imports https://docs.astral.sh/ruff/rules/unsorted-imports/
3. change is to == https://docs.astral.sh/ruff/rules/is-literal/
4. use list comprehension https://docs.astral.sh/ruff/rules/manual-list-comprehension/
5. change unused err to "_"
6. move some os to pathlib https://docs.astral.sh/ruff/rules/os-path-abspath

[CyberiaResurrection]

@even-even - Thanks for your pull request.

I am speaking subject to correction by @boxed and/or @Otto-AA - if they say otherwise, go with them - but I found how multiple ruff fixes were applied in each commit made it somewhat heavy going reviewing the PR so I could satisfy myself that the changes made did what you said they should do.

Ideally (for me, not necessarily anyone else), this PR would have been five, maybe six commits, with one ruff fix applied to test files per commit (the possible exception being removing the doubled up "import foo" line) - so the commit that had the "sort imports" fix applied didn't also have the "move os to pathlib" fix. The commit with "move os to pathlib" would be a separate commit, likewise with the "clean up newlines between methods" fix.

[Otto-AA]

Looks fine to me. I don't think enforcing styles makes much sense when there are only a few contributors. But I guess formatting from time to time makes sense.