Convert video quality test from a TEST_F to a TEST fixture.

The purpose is to make the fixture reusable in downstream
projects. The CL adds the following things to API:

- api/test/video_quality_test_fixture.h
- api/test/create_video_quality_test_fixture.h

The following things are moved to API:

- call/bitrate_constraints.h (api/bitrate_constraints.h)
- call/simulated_network.h (api/test/simulated_network.h)
- call/media_type.h (api/mediatypes.h)

These are required by the params struct passed to the
fixture. I didn't attempt to split the params struct into
an internal-only and public version in this CL, and as
a result we need to pull in the above things. They are
quite harmless though, so I think it's worth it in order
to avoid splitting up the test config struct.

This CL doesn't solve all the problems we need to
implement downstream tests; we probably need to upstream
tracing variants of FakeNetworkPipe for instance, but
that will come later. This puts in place the basic
structure for now.

Bug: None
Change-Id: I35e26ed126fad27bc7b2a465400291084f6ac911
Reviewed-on: https://webrtc-review.googlesource.com/69601
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23714}
diff --git a/video/screenshare_loopback.cc b/video/screenshare_loopback.cc
index 81bc346..19106d3 100644
--- a/video/screenshare_loopback.cc
+++ b/video/screenshare_loopback.cc
@@ -333,11 +333,11 @@
       flags::SelectedStream(), flags::NumSpatialLayers(), flags::SelectedSL(),
       flags::InterLayerPred(), SL_descriptors);
 
-  VideoQualityTest test;
+  auto fixture = rtc::MakeUnique<VideoQualityTest>(nullptr);
   if (flags::DurationSecs()) {
-    test.RunWithAnalyzer(params);
+    fixture->RunWithAnalyzer(params);
   } else {
-    test.RunWithRenderers(params);
+    fixture->RunWithRenderers(params);
   }
 }
 }  // namespace webrtc