This extends ruff testing to control/tests/*.py

I've tried to order the commits from obvious to subtle; on the whole:

• remove unused imports
• removed unused variables, or sometimes prefix with '_' where that seemed to make things clearer
• tweaked two eval calls

The remaining errors I'm less sure of. It would be simple to silence them with # noqa: F<relevant code>, and defer fixing these to later; then at least no new ruff errors will sneak in in the future.

More detail on remaining failures:

control/tests/kwargs_test.py

Here the dict literal has repeated keys for 'StateSpace.__init__' and 'StateSpace.__sample':

    'StateSpace.__init__':
        interconnect_test.test_interconnect_exceptions,
    'StateSpace.sample': test_unrecognized_kwargs,
    'NonlinearIOSystem.__init__':
        interconnect_test.test_interconnect_exceptions,
    'StateSpace.__init__': test_unrecognized_kwargs,
    'StateSpace.initial_response': timeresp_test.test_timeresp_aliases,
    'StateSpace.sample': test_unrecognized_kwargs,

Last-value-wins in dict literals, so the first two entries have no effect. It looks like this is just a check that something exists, so I guess we can just erase the first two entries?

control/tests/modelsimp_test.py

The variable mrk is assigned to: mrk = markov(outp, inp, 1, transpose=False). I didn't investigate issue #395 referenced above -- is it enough that this call doesn't raise?

control/tests/optimal_test.py

curved_seg_length is calculated, but not used. It's not obviously copy-and-pasted from another test, so I'm not sure if the test should be changed here. It's obviously easy enough to just remove this variable, and straight_seg_length just above it.