Refactor decorators

Split method bzbug into bzbug and get_bugzilla_bug. Add method
skip_if_bz_bug_open. get_bugzilla_bug is reponsible for fetching and caching
bug objects, bzbug can be used as usual, and skip_if_bz_bug_open causes the
currently running test to abort if the named bug is open.

The exact conditions under which tests will be skipped have changed. For
example, formerly, a test would skip if the relevant bug could not be fetched.
Now, the test will run if the relevant bug cannot be fetched.