autotest: Fix network_WiFi_BgscanBackoff to fail with a message

BUG=none
TEST=none

Change-Id: If06cdccc9c06db5dfa91b1d71b3ee8f130711cad
Reviewed-on: https://gerrit.chromium.org/gerrit/61129
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
1 file changed