Updating `pymsbayes.test.__init__`

Making all testing-level related functions methods of the
`TestLevel` class.

Also, adding `load_tests` function to override the behavior of the
default unittest test loader.
Updating `setup` in `setup.py`.

Explicitly directing `test_suite` to
`pymsbayes.test.get_unittest_suite`. This forces my test-loading
machinery to run and prevents the default behavior of loading all test
modules and tests that match the expectations of setup.
Updating test suite.

Adding `test_end_user.py`. This test module contains a suite of tests
for end users to run to ensure the bundled tools are working. The
default testing level is now `FAST`, in which case, the tests in
`test_end_user.py` are the only tests that get loaded and run.

Also, updating the run-level behavior of long-running tests throughout
the test suite to only run when the test level is `EXHAUSTIVE`.