test unsteady examples

[gdmcbain]

@kinnala, in reviewing #674 yesterday:

Can you figure out how this could be run in the test suite? In particular, tests/test_examples.py should ideally run all examples so that they won't so easily break in the future. Maybe add some call to evolve to tests/test_examples.py?

Yes, increasingly important as more initial value problems #531 are tackled.

Currently, we have

scikit-fem/tests/test_examples.py

Lines 147 to 149 in 5909601

	def runTest(self):
	import docs.examples.ex19 as ex # noqa
	# TODO improve

which will catch some things, but I think it'll be easy enough to test evolution, as suggested.

[gdmcbain]

I see that a few steady examples are in the same boat:

• TestEx17
• TestEx18
• TestEx24
• TestEx26

I think they're all mine too!

[gdmcbain]

Right, exx17, 18, e.g., had test-like print-statements in the scripts; I've moved those to tests.test_examples.