Improve behavior of --numprocesses=auto and --pdb

[jirikuncar]

I was getting annoyed by changing pytest -ff -n=auto ... command when I wanted to append only --pdb. With this PR one can use -n=auto and anytime simply append --pdb without getting the UsageError.

[nicoddemus]

LGTM, thanks @jirikuncar!

[jirikuncar]

@nicoddemus is it necessary to fix the AppVeyor failures [1]?

---

[1] https://ci.appveyor.com/project/pytestbot/pytest-xdist/builds/22311072/job/tpjds44asi1uy5cj

[nicoddemus]

Hi @jirikuncar,

Yeah, we need to fix it as it is part of our CI. I think this is due to recent changes on how UsageError is displayed (might be related to pytest-dev/pytest#4651 which was merged just a few hours before your build failed).

I think it is enough to change:

result.stdout.fnmatch_lines(
    ["usage: pytest.py *", "pytest.py: error: my_usage_error"]
)

to:

result.stdout.fnmatch_lines(
    ["*usage: pytest.py *", "pytest.py: error: my_usage_error"]
)

I will push a commit to that effect, let's see. 👍

[nicoddemus]

Fixed linting, should be OK now. 👍

[nicoddemus]

Thanks a ton @jirikuncar for the PR!

[blueyed]

Nice, thanks!