[features] Handle/test relative testpaths with changed cwd during collection

[blueyed]

Follow-up for https://github.com/pytest-dev/pytest/pull/4317/files#r231709712.

There is also invocation_dir already:

pytest/src/_pytest/config/__init__.py

Line 729 in 4c00552

self.invocation_dir = py.path.local()