FAFT: Use _sshd_test() to ensure DUT alive

The original _ping_test() caused the following RPC call failed in some state.
Using _sshd_test() is the correct way.

BUG=chromium-os:239034
TEST=None, trivial change, hard to repro the original failure case.

Change-Id: I5ee8c04145ed28b2db0c9c5692b4de89c229a3d5
Reviewed-on: https://gerrit.chromium.org/gerrit/50554
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Queue: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
1 file changed