Move the stage-tests over to the new test-infrastructure. This moves
the test invocation into `./test/run/test_stages.py`, so it is invoked
as part of the runtime-tests. Secondly, the test-data is stored in
./test/data/stages/ so the path is relative to
TestBase.locate_test_data().
While at it, this also drops the dynamic class modifications and instead
uses subTest(). This simplifies the code quite a bit and avoids
dynamically creating python code.
d584a1e225