Refactor decorators

Split function bzbug into bzbug and get_bugzilla_bug. Add method
skip_if_bz_bug_open.

* bzbug can be used as before.
* get_bugzilla_bug is reponsible for fetching and caching bug objects.
* skip_if_bz_bug_open causes the currently running test to abort if the named
  bug is open. (It raises unittest.SkipTest)

The exact conditions under which tests will be skipped have changed. Formerly,
is an error occurred while fetching information about a Bugzilla bug, the test
would be skipped. Now, the test is allowed to run.