Add a module for running DistArray tests.
Import `test` at the package level.
Rename test_runner to run_tests.py.
Simplify run_tests quite a lot.

Remove the ability to run specific tests individually, but this probably isn't needed here.
Return returncode from shell command.
Merge pull request #537 from enthought/feature/user-testing

Allow users to run the DistArray tests from the installed package