tests have unexpected side effects on each other

[singuliere]

Steps to reproduce:

• git checkout c4868e0
• tox -e py27-django111-- workbench/test/test_views.py # fails

E           PluginMissingError: xblockwithoutstudentview

• tox -e py27-django111 -- workbench/test/test_scenarios.py workbench/test/test_views.py # works