Remove references to /tmp in tests

This is to make progress towards supporting Solano or other CIs (e.g. jenkins running concurrent jobs per worker).

Including:

* Change path for mongo_metadata_inheritance to use unique temp dir
* Update SASS_CACHE_PATH with thread-safe temp dir
* Remove hard coded directory reference for grades download tests
* Remove hard coded directory reference for direct mongo test

@benpatterson @clytwynec @muhammad-ammar 
nitpick: move this import up, above the utils imports