Add new pytest_runtest_logfinish hook

[nicoddemus]

With the new hook I have been able to easily fix #3088 locally.

After this is merged, I will open the appropriate PR in pytest-xdist to trigger the new hook.

Fix #3101

[coveralls]

Coverage increased (+0.002%) to 92.611% when pulling b68b80a on nicoddemus:logfinish-hook-3101 into 93306f6 on pytest-dev:features.