Modernize WebRTC integration tests to support simulated time

- Introduce internal::PeerConnectionIntegrationTestBase to allow
  injecting TimeController and customizing thread creation.
- Add PeerConnectionIntegrationTestWithSimulatedTime for tests that run
  in simulated time.
- Refactor VirtualSocketServer and FirewallSocketServer management to
  use std::unique_ptr and make ownership explicit, avoiding leaks.
- Update FakeAudioCaptureModule to support running on a simulated
  thread. Note that a task queue does not work - switching to task queue
  caused timeouts on many bots
- Update various integration tests to use the new base classes and
  support simulated time.
- Add TODO for future unification of NewGetStats implementations
  (https://issues.webrtc.org/501310910).

Bug: webrtc:42223992
Change-Id: I15a23b30aad728e9fe2c890f5195bff46a6a6964
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462820
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47462}