Enable microsite tests

Move microsite test assets to common/test
Rename helper method to not have test in the name
Rename test css file

@chrisndodge @flowerhack 
We should merge this to master because right now `rake builddocs` is failing with:
22:21:36 **** Error loading microsite /home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/edx-platform/test_microsites/test_microsite. Directory does not exist