Expose NetEqDecodingTest for re-use in chromium tests.

This CL allows to trigger related tests when rolling opus
(at chromium side). Namely:
* TestOpusBitExactness
* TestOpusDtxBitExactness

This CL also prevents name clash for OpusTest:
* modules/audio_coding/test/opus_test.h: Helper class.
* modules/audio_coding/neteq/opus_unittest.cc: Local test fixture.

Bug: chromium:1002973
Change-Id: If8470b5f64fbdb1f7a84b838bde62d8c90390f2c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159033
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29759}

modules/audio_coding/codecs/opus/opus_unittest.cc

@@ -98,8 +98,6 @@ int SamplesPerChannel(int sample_rate_hz, int duration_ms) {
   return samples_per_ms * duration_ms;
 }
 
-}  // namespace
-
 using test::AudioLoop;
 using ::testing::Combine;
 using ::testing::TestWithParam;
@@ -150,6 +148,8 @@ class OpusTest
   const int decoder_sample_rate_hz_{std::get<4>(GetParam())};
 };
 
+}  // namespace
+
 // Singlestream: Try all combinations.
 INSTANTIATE_TEST_SUITE_P(Singlestream,
                          OpusTest,