Enable GN check for webrtc/{ortc,p2p}

Introduce new target //webrtc/p2p:rtc_p2p_test_utils to host
test-related utilities.
Previously uncovered header "base/fakecandidatepair.h" is now also in a target.

BUG=webrtc:6828

Review-Url: https://codereview.webrtc.org/2714263004
Cr-Commit-Position: refs/heads/master@{#17036}

.gn

@@ -43,6 +43,8 @@ check_targets = [
   "//webrtc/modules/video_coding/*",
   "//webrtc/modules/video_processing/*",
   "//webrtc/modules/remote_bitrate_estimator/*",
+  "//webrtc/ortc/*",
+  "//webrtc/p2p/*",
   "//webrtc/stats/*",
   "//webrtc/voice_engine/*",
 ]