Update integration tests to use pytest conventions

[Sherwin-14]

Integration tests have been failing silently for some time (#738). In the course of adding new integration tests (#736), we found some errors that were not reporting very much information, presumably either because the code that's generating the errors are not following pytest conventions.

1. Tests which import assertions and call for example assertions.assertTrue(x) should be updated to builtin assertions, for example assert x is True.
2. Tests which do setup at the module level should be updated to use fixtures