Use dedicated threads in RtpSenderReceiver unit tests

Update the RtpSenderReceiverTest fixture to instantiate separate worker
and network threads instead of using the current thread for all roles.
This change ensures that the tests more accurately reflect the actual
multi-threaded environment used in production.

Key modifications:
* Convert thread members to unique_ptrs and start them during setup.
* Wrap media channel interactions in BlockingCall to ensure they execute
  on the worker thread.
* Replace run_loop_.Flush() and WaitUntil poll loops with a Run/Quit
  pattern for async SetParameters to properly synchronize across
  different threads.
* Refactor RtpSenderReceiverDeathTest to use the current thread
  exclusively, as death tests often have limitations in multi-threaded
  environments.

Bug: webrtc:42222804
Change-Id: I67b6c2fb058bc49169790fd76d7236527d91de06
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/441480
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46628}