Add support for test method execution (GRADLE-2865)

Add support to specify list of test method names using `test.single` property, with the (optional) format: `-Dtest.single=TestClassName#method1,method2`.

I show all tests passing on `subprojects/plugins`. Played around with some real testng and junit test code and was able to successfully run only the method(s) specified. Also tested to make sure not specifying any test method names still does indeed run all test methods in the class.