Various tests want to configure engine properties such as pre-emption
timeout and heartbeat interval. Some don't bother to restore the
original values again afterwards. So, add a helper to make it easier
to do this.

v2: Fix for platforms with no pre-emption capability.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>