Testrunner: Kill iPhone Simulator after each test

It looks like ios-sim is not able to launch next test set until
iPhone Simulator is re-started. Forcing to kill it after each test.

Change-Id: I17aff14497eb1e7f48d07a7a7a245cc7fbc186a9
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>