Add frequency invariant load tracking test

Created by: bjackman

See commit message.

This test is quite simple so I've tried to make it an example of how we could design tests so that they can be used both from the command line as automated tests, and via notebooks as diagnostic/analysis tools. That means I've also added a couple of commits to make instantiating tests from notebooks easier. The interface for doing that is a bit bizarre (instantiate then setUpClass) but it works. The test/target configuration is also not inline in the notebook, instead the config comes from target.config. I personally like that, but it might be confusing invisible magic to someone new to LISA.