Updates rtp_sender_unitests to use separate thread for pacer calls.

Some classes such as RtpSenderEgress makes assumptions about which
threads (e.g. paced sender vs worker thread) call specific methods.
Unit tests currently are single threaded so these checks are
essentially noops.

This change uses a separate task queue for calls epected to be called
by the pacer, so that inconsistencies in thread can be detected early.

Bug: None
Change-Id: Ic0904304a67eb034033524e62306da34b9eab8b4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178602
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31628}

modules/rtp_rtcp/BUILD.gn

@@ -553,6 +553,7 @@ if (rtc_include_tests) {
       "../../rtc_base:rtc_base_tests_utils",
       "../../rtc_base:rtc_numerics",
       "../../rtc_base:task_queue_for_test",
+      "../../rtc_base/task_utils:to_queued_task",
       "../../system_wrappers",
       "../../test:field_trial",
       "../../test:mock_frame_transformer",