We want to extend our base-class to support extensions to
unittest.TestCase, so make sure we inherit from it.
Adjust all callers to no longer inherit from TestCase, since this is now
done automatically by TestBase.
We use comments in all other tests, rather than doc-strings. Convert the
os-release test to do the same. If we wanted doc-strings, we can convert
all tests over. This commit just tries to keep the tests in-sync.
Note that doc-strings cause `unittest` to print the doc-strings to
stdout during test-execution, making it overly verbose (especially for
multiline docs). By converting it to comments, this behavior is
suppressed.
Use the new `locate_test_data()` helper to get access to test-data.
Guard the test with `have_test_data()` to skip it in case test-data
access is not available.