Reduce the runtime of some ACM tests in modules_tests

webrtc/modules/audio_coding/test/TestVADDTX.cc

Index: webrtc/modules/audio_coding/test/TestVADDTX.cc
diff --git a/webrtc/modules/audio_coding/test/TestVADDTX.cc b/webrtc/modules/audio_coding/test/TestVADDTX.cc
index 98b1224762ddd6e72df426b9aaf776a2c302ed0a..6a0b270e27018853af815e864e96192ca748899f 100644
--- a/webrtc/modules/audio_coding/test/TestVADDTX.cc
+++ b/webrtc/modules/audio_coding/test/TestVADDTX.cc
@@ -12,6 +12,7 @@
 
 #include <string>
 
+#include "webrtc/base/checks.h"
 #include "webrtc/engine_configurations.h"
 #include "webrtc/modules/audio_coding/test/PCMFile.h"
 #include "webrtc/modules/audio_coding/test/utility.h"
@@ -87,6 +88,12 @@ void TestVadDtx::Run(std::string in_filename, int frequency, int channels,
   PCMFile in_file;
   in_file.Open(in_filename, frequency, "rb");
   in_file.ReadStereo(channels > 1);
+  static const int kTestLengthMs = 1000;
+  const int test_length_blocks = rtc::CheckedDivExact(kTestLengthMs, 10);
+  in_file.SetNum10MsBlocksToRead(test_length_blocks);
+  // Fast-forward both files 500 ms (50 blocks). The first second of the file is
+  // silence, but we want to keep half of that to test silence periods.
+  in_file.FastForward(50);
 
   PCMFile out_file;
   if (append) {