Simplify/clarify test suite by using pytest fixtures

[vtnate]

Expected Behavior

Tests should not duplicate effort, but also not silently require other tests to have already run.
We could use PyTest fixtures to call other tests and make this more error-proof for developers.

Actual Behavior

Some tests require others to be run, without notifying developers of that requirement. Other tests duplicate test code to not silently require another test.

Steps to Reproduce

Review tests in test_uo_des.py and test_gmt_lib.py