rostest: Allow test suites to be called in rostest.rosrun

See [this question](http://answers.ros.org/question/122786/any-way-to-use-a-testsuite-instead-a-testcase-with-rostest/).

The idea is to extend the functionality of `rostest.rosrun` so it can accept both `unittest.TestCase` and `unittest.TestSuite` in its parameters. 
