Adds xdist and cleans up some test namespace collisions

[boyanpenkov]

I had wanted to use https://pypi.org/project/pytest-xdist/ to run tests in parallel, and this PR does that.

This exposed a number of race conditions in the original test suite, which I cleaned up by forcing them to run in series -- using --loadfile forces them to run in series by file, so I placed the race-conditiony ones in a separate file: test_serial.py, so the speedup for now is not actually the worker-count. The next step for improving this is using xdist-groups to specify manually which tests should run together.

More importantly, since I had to run though five files worth of tests manually, I discovered that multiple tests were named the same, and consequentially were not being run. I fixed this, and the gross test count went up from 154 to 157 for free essentially.

Note that this was only tested on a codebase that assumes perrette#79 went in.

[boyanpenkov]

Just an observation for now -- this fails if you run with tox --parallel to try to do all the envs at the same time, for the same fundamental reason -- trying to write to the same bibtex files in the same places.

[boyanpenkov]

@perrette Please note this as well -- the existing test config, conda or not, had some duplicate tests and namespace collisions.