Run tests in parallel via tox for 3.5x speedup

[hugovk]

#9342 isn't ready to enable pytest-xdist across the board (CI and local) because of difficulties with leak tests.

But let's at least have the benefits for local testing via tox, if not in CI (which doesn't use tox).

Also add the test-arrow extra to enable some more tests.

pytest times on macOS with tox -e py314 on cached build:

Old table with test-arrow

	No test-arrow	With test-arrow
Sequential	41.23s	42.41s
Parallel	11.71s	13.39s
Speedup	3.52x	3.17x

	Time
Sequential	41.23s
Parallel	11.71s
Speedup	3.52x

Edit: removed the test-arrow extra because it fails on 3.15 due to missing wheel.

[radarhere]

Why wouldn't the leak failure happen locally?

[hugovk]

That's a good question! It'd be easier to investigate if it did happen locally :)

I just tried 10 runs of tox -e py314 and hit the spider KeyError we also had at #9342 (comment):

    def teardown_module() -> None:
>       del Image.EXTENSION[".spider"]
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
E       KeyError: '.spider'

So I've re-added that fix and 10 runs passed, followed by 10 x tox -e py310 that passed.