Delete TestListener and top-level thread wrapping.

Instead use rtc::AutoThread in tests that need that.

Bug: webrtc:9714
Change-Id: I1f33b1b2d321770d062504dd9ef86d66a345dd42
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/254681
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36950}
diff --git a/pc/sdp_offer_answer_unittest.cc b/pc/sdp_offer_answer_unittest.cc
index 2ce9e52..4f16de4 100644
--- a/pc/sdp_offer_answer_unittest.cc
+++ b/pc/sdp_offer_answer_unittest.cc
@@ -95,6 +95,7 @@
   rtc::scoped_refptr<PeerConnectionFactoryInterface> pc_factory_;
 
  private:
+  rtc::AutoThread main_thread_;
 };
 
 TEST_F(SdpOfferAnswerTest, OnTrackReturnsProxiedObject) {