'``InstallVirtualEnv.run`` installs a virtual python environment using
create_virtualenv passing ``target_path`` as ``root``.'

def test_run(self):

    virtualenv = VirtualEnv(root=FilePath(self.mktemp()))
    step = InstallVirtualEnv(virtualenv=virtualenv)
    calls = []
    self.patch(step, '_create_virtualenv', (lambda **kwargs: calls.append(kwargs)))
    step.run()
    self.assertEqual([dict(root=virtualenv.root)], calls)
