PYTHON-680 Add new test command to setup.py in order to avoid DeprecationWarning when raising SkipTest

The setuptools 'test' command uses the TestResult class from unittest,
even when we're using unittest2 for Python 2.6. Since the old unittest
TestResult class doesn't have an addSkip method, raising SkipTest
causes a DeprecationWarning instead of skipping a test.