Change which BZ bug states cause a test to skip

Make the `skip_if_bug_open` decorator skip a test if it points to a bugzilla bug
in any of the following states:

* NEW
* ASSIGNED
* POST
* MODIFIED

Formerly, only the 'NEW' and 'ASSIGNED' bugzilla bug states caused
`skip_if_bug_open` to skip a test.
:+1: