Support running tests in parallel

[bwohlberg]

Since our test suite takes quite a while to run, it would be convenient to be able to run the tests in parallel using either pytest-xdist or pytest-parallel. Unfortunately neither of these pytest plugins works at the moment

• pytest-xdist fails in the test collection phase (possibly related to Different tests were collected between gw0 and gw1 pytest-dev/pytest-xdist#432)
• pytest-parallel hangs in the test execution phase

[FernandoDavis]

The following are the only tests that do not work with x-dist:

• scico/test/test_admm.py
  Fails due to mismatch of results. Not sure if directly related to x-dist or if it's a mistake in my current build.
• scico/test/test_functional.py
  Different tests were collected between workers. This is likely due to concurrency issues in data between test classes/methods.

After I finish taking care of other issues I will proceed and fix these two modules so that they are compatible. Could be a big or small change though, will update this thread on findings.

[bwohlberg]

Closing for now since this is largely addressed in #390.