tests/sysfs: Update timeslice/preemption for new range limits

Guc submission imposes new range limits on certain scheduling
parameters. The idempotent sections of the timeslice duration and
pre-emption timeout tests was exceeding those limits and so would fail.

Reduce the excessively large value (654s) to one which does not
overflow (54s). Also add an assert that the write of the new value
actually succeeds.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>