s390:zdtm: Add test for tasks > 4TB

If the kernel contains patch ee71d16d22 ("s390/mm: make TASK_SIZE
independent from the number of page table levels") we are able
to checkpoint tasks > 4TB.

Add a testcase to verify this.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>