Remove usage of unittest in favor of pytest

[SamWitty]

In PRs #456 and #457 three additional test files were added to the test suite using the unittest python testing library. To maintain consistency with the existing tests, these tests should be removed in favor of pytest, which is used in the remainder of the repository. These new tests pass as pytest is compatible with tests written in unittest, but maintaining a single testing style will be easier to maintain as the scope of PyCIEMSS grows.

In addition, the additional testing introduced in #456 and #457 should be moved into the existing test_interfaces.py file using the existing test fixtures in fixtures.py. At present, the new tests introduce manual filepaths that are isolated from this "gold standard" collection of models in a way that will be difficult to maintain.