adding extfs unit test

Hi @jfindlay / @rallytime , Please review the kmod_test case and let me know if you have any review comments.
This needs a `@skipIf(NO_MOCK, NO_MOCK_REASON)` above the class.

`NO_MOCK` and `NO_MOCK_REASON` must be imported from `salttesting.mock` along with `MagicMock` and `patch`.