CRAS: test_client - add ability to test loopback streams.
CRAS: test_client - add ability to test loopback streams.

cras_test_client can now accept a '--loopback_file' parameter.  It
will record from the system loopback stream to the specified file.
This can be used while playing output from other clients to get the
post-mix audio stream.

BUG=chromium:247160
TEST=run this test

Change-Id: I442a729f26dec541fc2ec480c10c404dff6ef953
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/60242
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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