Attempt to speed up tests with xdist.

[jsirois]

No description provided.

[jsirois]

This does speed up tests on OSX modestly, perhaps 10-15%. On my machine it speeds things up by ~60%. Perhaps more importantly this caught a bug in Pants pre-Pex 2.x:

...
New virtual environment successfully created at /Users/travis/tmp/popen-gw2/test_pants_1_260/PANTS_HOME/bootstrap-Darwin-x86_64/1.26.0.dev0_py36.
20:14:56 [INFO] Resolving new plugins...:
  pantsbuild.pants.contrib.go==1.26.0.dev0
/Users/travis/tmp/popen-gw2/test_pants_1_260/PANTS_HOME/bootstrap-Darwin-x86_64/1.26.0.dev0_py36/lib/python3.6/site-packages/pex/pep425tags.py:274: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
ERROR: [Errno 66] Directory not empty: '/Users/travis/.cache/pants/plugins/tmpubx60nc2' -> '/Users/travis/.cache/pants/plugins/twitter.common.log-0.3.11-py3-none-any.whl-install'
...

This should not be a problem in pantsbuild.pants>=1.26.0.dev1 since it uses Pex 2+ which includes pre-installation of wheels in directory chroots and it does so via a multi-process concurrency safe atomic directory primitive.

Another - not sure it's worth it change.

[jsirois]

I'll leave the CI shard with the concurrency error red until you review. If you like it, then I can click retry and we'll have known flaky <=1.26.0.dev0 Pants executions.

[Eric-Arellano]

Looks good. Thanks for making so many improvements to this repo recently!

[Eric-Arellano]

then I can click retry and we'll have known flaky <=1.26.0.dev0 Pants executions.

Your call on if we're okay with the flakes. I was already planning on no longer testing <= 1.26.0 so that we can guarantee pants.toml support.

[jsirois]

@Eric-Arellano it's probably best you try this on OSX 1st. /tmp/... appears to get realpathed by the time watchman sees it and under OSX that leads to a too long pathname. So if tox -etest fails for you with too long a pathname, this PR is no good without a solution to OSX shenanigans.

[benjyw]

Closing this due to #142 - we will get test concurrency via Pants once that is merged.