Move one CircularBuffer to webrtc::test namespace.

There are currently two webrtc::CircularBuffers defined:
- modules/audio_coding/test/utility.{h,cc}
- modules/audio_processing/echo_detector/circular_buffer.{h,cc}

This CL moves the former definition to the webrtc::test namespace,
to avoid link errors in a future build target.

BUG=None

Review-Url: https://codereview.webrtc.org/2667383008
Cr-Commit-Position: refs/heads/master@{#16553}
diff --git a/webrtc/modules/audio_coding/test/utility.cc b/webrtc/modules/audio_coding/test/utility.cc
index 5c1fc3f..30bafd4 100644
--- a/webrtc/modules/audio_coding/test/utility.cc
+++ b/webrtc/modules/audio_coding/test/utility.cc
@@ -136,6 +136,8 @@
 
 }
 
+namespace test {
+
 CircularBuffer::CircularBuffer(uint32_t len)
     : _buff(NULL),
       _idx(0),
@@ -265,6 +267,8 @@
   }
 }
 
+}  // namespace test
+
 bool FixedPayloadTypeCodec(const char* payloadName) {
   char fixPayloadTypeCodecs[NUM_CODECS_WITH_FIXED_PAYLOAD_TYPE][32] = { "PCMU",
       "PCMA", "GSM", "G723", "DVI4", "LPC", "PCMA", "G722", "QCELP", "CN",