shared.cfg: Check SSDT for both S3/S4 tests

The SSDT shows whether the bios set the suspend status
correctly, so it's another test point for both S3/S4
tests.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>

Tested guest_suspend..guest_enable/disable_s3/s4 for Fedora18 and RHEL6.4 guests, all passed.