Switch peer_connection_encodings_integrationtest to WaitUntil

Demonstrates use of matchers and WaitUntil to have tests that are more
understandable during failure.

Drive by changes,
* Remove the `const` on RTCStats.id_ as to allow for the implicit copy
constructor.
* Add [[nodiscard]] to WaitUntil as it is not useful without checking
the return value.

Bug: webrtc:381524905
Change-Id: I379910ce0fc8d9d81c96b8f164aa5a040637c85a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/370802
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Auto-Submit: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43599}

api/stats/rtc_stats.h

@@ -103,7 +103,7 @@ class RTC_EXPORT RTCStats {
   virtual std::vector<Attribute> AttributesImpl(
       size_t additional_capacity) const;
 
-  std::string const id_;
+  std::string id_;
   Timestamp timestamp_;
 };
 

pc/BUILD.gn

@@ -2414,12 +2414,14 @@ if (rtc_include_tests && !build_with_chromium) {
       "../rtc_base:threading",
       "../rtc_base:timeutils",
       "../rtc_base:unique_id_generator",
+      "../rtc_base/containers:flat_map",
       "../rtc_base/synchronization:mutex",
       "../rtc_base/third_party/base64",
       "../rtc_base/third_party/sigslot",
       "../system_wrappers:metrics",
       "../test:rtc_expect_death",
       "../test:run_loop",
+      "../test:scoped_key_value_config",
       "../test:wait_until",
       "../test/pc/sctp:fake_sctp_transport",
       "//testing/gtest",