CRAS: test_client - Truncate the capture file at opening.
CRAS: test_client - Truncate the capture file at opening.

When writing to an exsiting file, we should truncate the file before writing
new audio samples.

BUG=NONE
TEST=Run the program on parrot

Change-Id: Ifbf7913882a3c80a2a818dae33579e795f021dc0
Reviewed-on: https://chromium-review.googlesource.com/177990
Reviewed-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>
Commit-Queue: Owen Lin <0a54d949e4b0b01f23b9b06ccd3e0b748fb4eb25@chromium.org>
Tested-by: Owen Lin <0a54d949e4b0b01f23b9b06ccd3e0b748fb4eb25@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd