Split out this setup.py fix from https://github.com/pypa/virtualenv/pull/635

Fix default args for py.test to be "tests" instead of using the
command name "test" also as the directory name to find tests.
See http://pytest.org/latest/goodpractises.html#integration-with-setuptools-test-commands - should this not be ```self.pytest_args = []```?

Having said this, the tests for ```virtualenv``` are a bit of a mess and don't really follow the ```py.test``` standard discovery rules - see http://pytest.org/latest/goodpractises.html#test-discovery

I'd rather we fixed up the tests properly instead of hard-coding a ```tests``` directory that could cause us grief later...