iOS: Add resource files for tests and implement OutputPath

With this change the following tests have been successfully
passing in the iOS Simulator for iPhone 5 and iOS 9:
* audio_decoder_unittests
* common_video_unittests
* modules_tests
* rtc_api_objc_tests
* rtc_pc_unittests
* system_wrappers_unittests
* voice_engine_unittests

The modules_unittests and common_audio_unittests are
handled in https://codereview.webrtc.org/1698033002/

BUG=webrtc:4755
NOTRY=True

Review URL: https://codereview.webrtc.org/1694353003

Cr-Commit-Position: refs/heads/master@{#11646}

webrtc/modules/video_coding/test/test_util.cc

@@ -26,7 +26,7 @@ CmdArgs::CmdArgs()
       width(352),
       height(288),
       rtt(0),
-      inputFile(webrtc::test::ProjectRootPath() + "/resources/foreman_cif.yuv"),
+      inputFile(webrtc::test::ResourcePath("foreman_cif", "yuv")),
       outputFile(webrtc::test::OutputPath() +
                  "video_coding_test_output_352x288.yuv") {}