Skiptests

Change how tests are skipped. Rather than checking to see whether a test should be skipped when the test function is defined, check to see whether a test should be skipped when it executes.
Not a big fan of having multiple return statements for a method/function... how about:

```python
try:
    bug = get_bugzilla_bug(bug_id)
    # If bug is open, skip the test.
    if bug.status in BUGZILLA_SKIP_TEST_STATUSES:
        raise unittest.SkipTest('Skipping test. Reason: {}'.format(bug))
except BugFetchError as err:
    # Could not fetch bug report. Allow test to proceed.
    logging.warning(err.message)

return None
```