Add xdist test

[andyg0808]

Create a test to check that a version of #36 can be achieved as-is.

By telling xdist to keep tests from one file together on one thread using --dist=loadfile, we can make sure that dependent tests are executed on the same thread, as long as they're in the same file.

[ftobia]

Hm, I'm (finally) getting around to take a look at this. I have it failing for me locally, here's the captured stderr call

tests/test_xdist_handling.py:62: AssertionError
------------------------------------------------------------------------------- Captured stderr call --------------------------------------------------------------------------------
ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: -n3 --dist=loadfile
  inifile: None
  rootdir: /private/var/folders/wq/ltnkdtv94t5df_69nk9243f80000gn/T/pytest-of-frank/pytest-2/test_xdist_ordering0

[MRigal]

@ftobia Maybe you haven't update your local env? testenv now has an additional dependency

[ssbarnea]

This is a critical test to have. xdist is one of the most popular plugins that can heavily conflict with ordering and we need to assure no regression happens or to fail-fast when we detect incompatible use-cases.