documented testing command workflow

[cchall]

Some notes from discussing testing with @ncook882 .
Using pytest in rsopt/tests results in some known failure cases:

• test_setup.py has some tests that produce errors due to the way unittest.TestCase.setUp runs unless each test case is done individually
• regression_tests needs to use for subTest to fully function

A single script that executes the testing workflow should be a part of tests/