Remove unused function `is_bzbug`
Replace bzbug with skip_if_bz_bug_open

Decorator skip_if_bz_bug_open can be used just like bzbug. However, there are
several advantages to using the new decorator.

* The new decorator is better named.
* The new decorator does not trigger calls to a bugzilla server when it is
  evaluated. Instead, calls to a bugzilla server are delayed until the test
  suite is executed.
* The new decorator skips tests under slightly different conditions. Formerly,
  if an error occurred while fetching information about a Bugzilla bug, the
  test would be skipped. Now, the test is allowed to run.

Finally, function _get_bugzilla_bug has been created. It is reponsible for
fetching and caching bug objects.
Fix numerous broken function calls

Do not call `bzbug`. Instead, call `skip_if_bz_bug_open`.
Replace redmine with skip_if_rm_bug_open

Decorator skip_if_rm_bug_open can be used just like redmine. However, the same
advantages that apply to using skip_if_bz_bug_open apply to this new decorator
too.
Fix numerous broken function calls

Do not call `redminebug`. Instead, call `skip_if_rm_bug_open`.
Clean up import statements
Merge pull request #889 from Ichimonji10/skiptests3

Lazily choose whether to skip tests