Use separate threads in PeerConnectionInterface tests

Update PeerConnectionInterface unit tests to use dedicated threads for
signaling, worker, and network operations. This transition moves away
from using the current thread for all roles, better simulating real-
world usage and satisfying thread-affinity requirements.

Key modifications:
* Initialize and start explicit network (with a socket server) and
  worker threads during test setup.
* Wrap PortAllocator initialization and access within BlockingCall
  sequences on the network thread to comply with thread-checker
  constraints.
* Update media track and stream creation helpers to utilize the
  designated worker thread.
* Refactor the test PeerConnectionFactory to accept and manage the
  lifecycle of these separate threads.

Bug: webrtc:42222804
Change-Id: Ie90bf14dd5bbe1771e13096d3eceabe7b54fb2df
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/441500
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46629}