tests: run in parallel

[henryiii]

You can run in serial with -- -n0.

Nice little timesave in CI, but have to make sure tests are isolated (such as ones that fight for making a .nox dir in the same place).

For some reason, this seems to sometimes corrupt coverage. Needs to be reliable before we can merge.

[henryiii]

Conda on windows:

75.29s call     tests/test_virtualenv.py::test_condaenv_create
45.79s call     tests/test_virtualenv.py::test_reuse_conda_environment
42.85s call     tests/test_virtualenv.py::test_condaenv_create_interpreter
32.60s call     tests/test_virtualenv.py::test_stale_environment[conda-virtualenv-False]
29.08s call     tests/test_virtualenv.py::test_condaenv_detection

Wow...