Consolidate the CSS classes tests into the TriggerTestsMixin, to make them easier to keep up to date and to enable use elsewhere (e.g. student_groups).
Consolidate the CSS classes tests into the TriggerTestsMixin, to make them
easier to keep up to date and to enable use elsewhere (e.g. student_groups).

Tested:
* pylint CLEAN
$ python scripts/project.py --test=modules.courses
  2016/10/03 14:49:43	Progress so far: 0 failed, 16 completed, 0 running, 0 pending.
  2016/10/03 14:49:43	Ran 117 tests in 16 test classes; took 49s
$ python scripts/project.py --test=modules.explorer
  2016/10/03 12:42:26	Progress so far: 0 failed, 2 completed, 0 running, 0 pending.
  2016/10/03 12:42:26	Ran 9 tests in 2 test classes; took 13s
$ python scripts/project.py --test=modules.student_groups
  2016/10/03 12:44:58	Progress so far: 0 failed, 13 completed, 0 running, 0 pending.
  2016/10/03 12:44:58	Ran 84 tests in 13 test classes; took 71s

	Change on 2016/10/03 by tlarsen <tlarsen@google.com>

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135015075

andela-angene/coursebuilder-core,andela-angene/coursebuilder-core,andela-angene/coursebuilder-core,andela-angene/coursebuilder-core