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.