added '-sdk iphonesimulator' xctool parameter in tests_module.rb

'-sdk iphonesimulator' parameter is needed for unit tests. For some reason current xctool refuses to run them without it, and complains if you don't provide it:

  run-test ParsersTests.octest (iphoneos6.1, logic-test, GC OFF)
  failed: Testing with the 'iphoneos' SDK is not yet supported.  Instead, test with the simulator SDK by setting '-sdk iphonesimulator'.
** TEST FAILED: 0 of 0 tests passed ** (3332 ms)