Decode stderr after executing a replay

The touch tests were showing stderr as a binary string when run in
Python 3,

BUG=none
TEST=run an erroring touch test using Python 3 and check the stderr

Change-Id: I9f547766dffa99e7be4c2a5ed6b6442524aeb482
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2166455
Tested-by: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
1 file changed