Test examples in CI

> I guess the examples aren't run as part of CI? examples/run_examples.sh was supposed to help with that.

We should add an appropriate examples_test.py [like we do in Slycot](https://github.com/python-control/Slycot/blob/master/slycot/tests/test_examples.py)

_Originally posted by @bnavigator in https://github.com/python-control/python-control/issues/518#issuecomment-766342661_