TST: Use unittest consistently in tests

[carterbox]

Purpose

Some test skips were implemented using the pytest module instead of unittest. This would cause the tests to not be skipped if the test file was run directly.

Approach

Replace all calls to pytest.mark.skip with unittest.skip.

Pre-Merge Checklists

Submitter

• [x] Write a helpfully descriptive pull request title.
• [x] Organize changes into logically grouped commits with descriptive commit messages.
• [x] Document all new functions.
• [x] Write tests for new functions or explain why they are not needed.
• [ ] Build the documentation successfully
• [x] Use yapf to format python code.

Reviewer

• [ ] Actually read all of the code.
• [ ] Run the new code yourself.
• [ ] Write a summary of the changes as you understand them.
• [ ] Thank the submitter.