audio_streams: Cache elapsed Duration in unit tests

Cache start_time.elapsed() with `elapsed` to prevent

"overflow when subtracting duration" error.

BUG=chromium:1161663
TEST=`cargo test`

Change-Id: I1178e1d1a26471b0695b771f6687a806cf284600
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/2612305
Tested-by: Chih-Yang Hsia <paulhsia@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
Commit-Queue: Chih-Yang Hsia <paulhsia@chromium.org>
2 files changed