Fix flakky test: ensure that DVQA was executed for some time.

It will ensure that clock moved forward, so clock related metrics as
CPU usage will be also calculated correctly. We should dig into it deeper
to fix the root cause later.

Example failure:
https://ci.chromium.org/ui/p/webrtc/builders/try/win_asan/44610/overview

Bug: b/205824594
Change-Id: If1ebcf6a2b88ba0054479be292cca8f50506e741
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/238381
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35379}

test/pc/e2e/analyzer/video/default_video_quality_analyzer_test.cc

@@ -1459,6 +1459,11 @@ TEST(DefaultVideoQualityAnalyzerTest, GetStreamFrames) {
                             VideoQualityAnalyzerInterface::EncoderStats());
   }
   // We don't need to receive frames for stats to be gathered correctly.
+
+  // Give analyzer some time to process frames on async thread. The computations
+  // have to be fast (heavy metrics are disabled!), so if doesn't fit 100ms it
+  // means we have an issue!
+  SleepMs(100);
   analyzer.Stop();
 
   EXPECT_EQ(analyzer.GetStreamFrames(), stream_to_frame_ids);