[testing] Use shlex to be able to process quote enclosed strings as parameters

This makes it possible to pass arguments like "-Grid.Cells "0 0 0"" via CMake and the runtest script.

shlex is in the standard library (https://docs.python.org/3/library/shlex.html / https://docs.python.org/2/library/shlex.html) so that this doesn't add a bad dependency.