Further refactor skip_if_bug_open

Move bug lookup logic out of `skip_if_bug_open` and into two new functions:

* `bz_bug_is_open`
* `rm_bug_is_open`

Create a solid battery of unit tests for these functions. Refactor existing,
nearby unit testing code.
Skip known failing tests

Use new method `bz_bug_is_open` to skip several known-failing tests. These tests
take advantage of the DDT module, and only some of the data passed in by the
`@ddt.data` decorator causes the test to fail. As a result, the usual
`@skip_if_bug_open` decorator is unsuitable, as it cannot selectively skip some
of the test generated by `@ddt.data`.
Merge pull request #1157 from Ichimonji10/bug-decorator-2

Further refactor skip_if_bug_open, skip failing tests