Fix unique key violation error in tests

[pxwxnvermx]

This PR fixes the unique key violation error that randomly occurs in the CI test runner. Using the django_get_or_create meta option for OpportunityAccessFactory, we can specify to run the get_or_create method to create objects when certain fields are present, in this case, user and opportunity. This will help avoid creating duplicate OpportunityAccess for users.

Failing CI Run and Test Description

[calellowitz]

Do you know how this happens? I would expect tests clean up state and new invocations to always have unique values. Generally we would probably want to know if tests were reusing objects or not, so I would prefer if we could figure out how those are getting repeated, although this is probably a fine change