[autotest] Harden the implementation of servo repair.

After using servo to install from USB, we shut down the target with
`self.run('halt')`.  In rare cases, the ssh command can fail to
terminate cleanly before the target halts, leading to a long timeout
and potentially a spurious failure.

This changes to use a more robust invocation of halt that shouldn't
suffer from the race to terminate cleanly.

BUG=None
TEST=tested in conjunction with CL:282766

Change-Id: I72b83e8c035ae659be2cac18afc9adba1bebf171
Reviewed-on: https://chromium-review.googlesource.com/294842
Reviewed-by: Paul Hobbs <phobbs@google.com>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
1 file changed