Remove unnecessary use of nosetests

Many of the tests invoked `nose.run()` in their `if __name__ == "__main__"` clause, making it difficult to run tests individually as a script (the entire test suite would run, instead of a single module).  Most of the tests are standard `unittest` classes anyway, so `unittest.main()` will suffice, with no loss of discoverability under `nose`.  For a couple of tests that don't use `unittest`, I just changed the runner to `nose.runmodule()` so it will only run the tests in that module if called as a script.

This also removes an old `SkipTest` for a test that doesn't seem to fail anymore (ref: https://svn.enthought.com/enthought/ticket/1618).