Refactor rtc_unittests into several targets.

Also fix some warnings.

NOTRY=True
BUG=webrtc:6954

Review-Url: https://codereview.webrtc.org/2611663002
Cr-Commit-Position: refs/heads/master@{#15900}

webrtc/sdk/objc/Framework/UnitTests/avformatmappertests.mm

@@ -160,7 +160,7 @@ TEST(AVFormatMapperTest, SuportedCricketFormats) {
       webrtc::GetSupportedVideoFormatsForDevice(mockDevice);
 
   // then
-  EXPECT_EQ(1, result.size());
+  EXPECT_EQ(1u, result.size());
 
   // make sure the set has the expected format
   EXPECT_EQ(expectedFormat, *result.begin());