Relanding: Remove rtc_p2p_unittests from ortc_unittests and rtc_media_unittests

These tests are already built into rtc_unittests, so they end up being
run three times. Fixed by creating a "p2p_test_utils" target that
contains the test utils that ortc_unittests and rtc_media_unittests
depend on, but not the tests themselves.

BUG=None
TBR=kjellander@webrtc.org

Review-Url: https://codereview.webrtc.org/2820263004
Cr-Commit-Position: refs/heads/master@{#17752}

webrtc/ortc/BUILD.gn

@@ -78,8 +78,8 @@ if (rtc_include_tests) {
       "../base:rtc_base_approved",
       "../base:rtc_base_tests_utils",
       "../media:rtc_unittest_main",
+      "../p2p:p2p_test_utils",
       "../p2p:rtc_p2p",
-      "../p2p:rtc_p2p_unittests",
       "../pc:pc_test_utils",
       "../system_wrappers:metrics_default",
     ]