Move iOS tests to XCTest from gtest.

tools-webrtc/ios/tests/common_tests.json

 {
   "tests": [
     {
+      "app": "apprtcmobile_tests",
+      "xctest": true
+    },
+    {
       "app": "audio_decoder_unittests"
     },
     {
       "app": "common_audio_unittests"
     },
     {
       "app": "common_video_unittests"
     },
     {
       "app": "modules_tests"
     },
     {
       "app": "rtc_pc_unittests"
     },
     {
       "app": "system_wrappers_unittests"
     },
     {
       "app": "test_support_unittests"
     },
     {
       "app": "voice_engine_unittests"
     }
   ]
 }