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`.