keyframe_test: use a fixed speed step for realtime

The lower complexity modes may not generate a keyframe automatically.
This behavior was found when running under Valgrind, as the slow
performance caused the speed selection to pick lower complexities than
when running natively. Instead, use a fixed complexity for the
realtime auto keyframe test.

Affected tests:
  AllModes/KeyframeTest.TestAutoKeyframe/0

Change-Id: I44e3f44e125ad587c293ab5ece29511d7023be9b

test/encode_test_driver.cc

@@ -109,6 +109,7 @@ void EncoderTest::RunLoop(VideoSource *video) {
       again = video->img() != NULL;
 
       PreEncodeFrameHook(video);
+      PreEncodeFrameHook(video, &encoder);
       encoder.EncodeFrame(video, flags_);
 
       CxDataIterator iter = encoder.GetCxData();