Add test to ensure _pytest is warning-clean on import

[asottile]

supersedes #4510

[asottile]

from a timing perspective, this test is less worse than I expected:

$ pytest testing/test_meta.py 
============================= test session starts ==============================
platform linux -- Python 3.6.8, pytest-5.2.1.dev5+g9a4c0b991, py-1.8.0, pluggy-0.13.0
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/tmp/x/pytest/.hypothesis/examples')
rootdir: /tmp/x/pytest, inifile: tox.ini
plugins: hypothesis-4.38.1
collected 55 items                                                             

testing/test_meta.py ................................................... [ 92%]
....                                                                     [100%]

============================== 55 passed in 6.04s ==============================

[nicoddemus]

Can you please rebase so we can get a green CI before merging? Thanks!