New pacer: keepalive fix, unittests coverage

This CL fixes a bug affecting the new PacedSender code path (not yet
active): keepalive packets will always be sent, even if sending is
paused or network is congested.

Mainly though, it refactors all the PacedSender unittests and makes
sure all of them (as applicable) are run using both the old and the new
code paths.

Bug: webrtc:10633
Change-Id: I531939b719a11147a2a7dbab77dfcccaf7a42d83
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/145721
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28578}

modules/pacing/paced_sender.cc

@@ -338,13 +338,16 @@ void PacedSender::Process() {
       critsect_.Enter();
       OnPaddingSent(bytes_sent);
     } else {
+      size_t keepalive_bytes_sent = 0;
       critsect_.Leave();
       std::vector<std::unique_ptr<RtpPacketToSend>> keepalive_packets =
           packet_router_->GeneratePadding(1);
-      critsect_.Enter();
       for (auto& packet : keepalive_packets) {
-        EnqueuePacket(std::move(packet));
+        keepalive_bytes_sent += packet->payload_size() + packet->padding_size();
+        packet_router_->SendPacket(std::move(packet), PacedPacketInfo());
       }
+      critsect_.Enter();
+      OnPaddingSent(keepalive_bytes_sent);
     }
   }