Add a test of EC power button handling

This test checks for the following functionality of EC power button
handling:
    - Short press wake system from G3 (shutdown for more than 10 sec)
    - Short press wake system from S5 (shutdown for less than 10 sec)
    - Long press wake system from G3 (shutdown for more than 10 sec)
    - Long press wake system from S5 (shutdown for less than 10 sec)
    - Long press (6s) to shut down system
    - Longer press (10s) to shut down system when kernel 'powerd' is
      disabled

BUG=chrome-os-partner:10279
TEST=Test passed.

Change-Id: Iae3992bcf48eaa769d2b35788519529da50ab83c
Reviewed-on: https://gerrit.chromium.org/gerrit/25384
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
2 files changed