Register the markers 'slow' and 'valgrind' in the `conftest.py` file
instead of `pytest.ini` as the latter file is not always present.

Use standard pytest markers everywhere in the numpy tests. At this point
there should be no nose dependency. However, nose is required to test
the legacy decorators if so desired.

At this point, numpy test cannot be run in the way with runtests, rather
installed numpy can be tested with `pytest --pyargs numpy` as long as
that is not run from the repo. Run it from the tools directory or some
such.

The renamed C modules provide low level functions for testing. The
rename marks them as private functions and makes them invisible to
pytest by default.