Separate build targets for APM bit exactness tools from unittests.

This places the bit exactness testing tools in audioproc_test_utils,
and removes it from audio_processing_unittests.

Bug: webrtc:8240
Change-Id: I6f54ea3c49c0212888c6f8a779ecc886d1d2baba
Reviewed-on: https://chromium-review.googlesource.com/663545
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#19815}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d57f9ddfd154d1a6ae4591fadd3493730b028865