Cleanup of test_node.py. (#1153)

These tests weren't always properly cleaning up after
themselves, and were sometimes improperly using the
setUp{Class} fixtures.  They also didn't always clean up
between tests, so sometimes the same node was reused
over and over again.  Clean all that up here, which
should make the tests much more robust.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>