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/audio_coding/neteq/audio_decoder_unittest.cc

@@ -96,9 +96,9 @@ double MseInputOutput(const std::vector<int16_t>& input,
 class AudioDecoderTest : public ::testing::Test {
  protected:
   AudioDecoderTest()
-      : input_audio_(webrtc::test::ProjectRootPath() +
-                         "resources/audio_coding/testfile32kHz.pcm",
-                     32000),
+      : input_audio_(
+            webrtc::test::ResourcePath("audio_coding/testfile32kHz", "pcm"),
+            32000),
         codec_input_rate_hz_(32000),  // Legacy default value.
         encoded_(NULL),
         frame_size_(0),