Re-work unit commitment tests

[bknueven]

Most unit commitment tests are regression-style tests and are generally too large, and hence take a bit of time to run. The unit commitment tests should be re-worked to have the following features:

1. Testing one constraint or component (e.g., minimum up time constraints, production cost, initial condition constraints)
2. Small; 1-3 generators at most, 1-3 buses at most, no more than 12 time periods.
3. Have an obvious solution which can be worked out easily by hand.
4. New/existing test cases should be auto-discovered by the test harness and not hard-coded.

We could keep some flagged regression tests (probably based of the current suite, or pglib-uc) as part of the re-work.