The unit test added in MDL-53000 makes assumptions about the plugins installed based on a vanilla install, which causes issues when 3rd party assignsubmission or assignfeedback plugins are installed.

The same problem is present in tests added by MDL-52924 for M3.1, where it assumes the only submission and feedback plugins returned in an array for a websvc call are core ones.

This is remedied by ensuring we're testing the known plugins where we know what to expect (that is, core plugins, not 3rd party).