Where it's clear that function is called for side effects (e.g., in a
q`with pytest.raises` block), don't assign function output.  Where
it's not clear, e.g., binary ops on LTI objects, call result `_sys` or
similar.  There are plenty of in-between cases: for those I chose
based on understandability.

Where imports were test fixtures, replaced with
`@pytest.mark.usefixtures('nameoffixture')`.

Provide relevant symbols via `locals` argument to eval.

Symbol reference in lambda which is never called.

The test had no assertions, but have been intended to test MIMO
feedback; for this see testMIMOfb in same file.

The function doesn't exist, but the test is never called.

slycontonly is not a fixture.

It looks like a block of code was copied-and-pasted between two test
functions; removed the unused variable in each case.