Add test coverage for P2PTransportChannel refactor (#8/8)

This converts all P2PTransportChannel unit tests to parameterized tests, with a string parameter for the field_trials which is used to enable the refactor. This adds a variation of each existing test using the refactored code path.

Tests are initialized twice, once for legacy and refactored path each, to strike a balance between file name length and descriptiveness.

Bug: webrtc:14367, webrtc:14131
Change-Id: I0469550d571ed389804eb486fe5bd22504e59373
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/275303
Commit-Queue: Sameer Vijaykar <samvi@google.com>
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38162}

p2p/BUILD.gn

@@ -297,6 +297,7 @@ if (rtc_include_tests) {
       "../rtc_base:rtc_base_tests_utils",
       "../rtc_base:socket",
       "../rtc_base:socket_address",
+      "../rtc_base:stringutils",
       "../rtc_base:testclient",
       "../rtc_base:threading",
       "../rtc_base:timeutils",