tests: fix "FD leakage detected" / turn --lsof into error again

[blueyed]

There are some "FD leakage detected" errors in builds, and pypy appears to has a lot of them: https://travis-ci.org/pytest-dev/pytest/jobs/420632250#L658

I think CI should fail (at least in one job) in case this happens, i.e. on any warning.

[RonnyPfannschmidt]

This breaks builds over things we have no control over

[nicoddemus]

... on any warning.

Btw we already configure to error on warnings, it is just that we put some of them that we don't have control over in a whitelist in tox.ini

[blueyed]

Just noticed that there are likely more with pypy due to the garbage collector being called explicitly?!
https://github.com/blueyed/pytest/blob/84e4909f5bd505a88b8ddf7e0aee191de0f3f631/src/_pytest/pytester.py#L112-L113

[blueyed]

@nicoddemus
This one is not in there though - but it is also not a real warning, but emitted by pytest itself (IIRC): https://github.com/blueyed/pytest/blob/84e4909f5bd505a88b8ddf7e0aee191de0f3f631/tox.ini#L207-L218

[blueyed]

Ok, --lsof was added in d4fe273 initially, to report and fail on FD leakage, but then later was turned into a warning (see #2366).
I think there is some real issue here, and that it does not make sense to run it for all tests, if it is only ignored then in the end (it is two subprocesses on top, and the explicit GC for pypy might also be the reason for pypy being slower in general, and especially with coverage tracking).