GN: Add target for modules_tests.

Additional changes I needed to make it work:

- Modified a header in RTPFile.cc. Every other file is
  using "webrtc/engine_configurations.h" instead.

- Disabled flag 4373 for msvs because it was disabled
  in build/common.gypi.

BUG=webrtc:6038
TBR=kwiberg@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2187563005
Cr-Commit-Position: refs/heads/master@{#13628}
diff --git a/webrtc/modules/audio_coding/test/RTPFile.cc b/webrtc/modules/audio_coding/test/RTPFile.cc
index 6077717..2ffe3da 100644
--- a/webrtc/modules/audio_coding/test/RTPFile.cc
+++ b/webrtc/modules/audio_coding/test/RTPFile.cc
@@ -20,7 +20,7 @@
 #endif
 
 #include "audio_coding_module.h"
-#include "engine_configurations.h"
+#include "webrtc/engine_configurations.h"
 #include "webrtc/system_wrappers/include/rw_lock_wrapper.h"
 // TODO(tlegrand): Consider removing usage of gtest.
 #include "testing/gtest/include/gtest/gtest.h"