tools/testing: Add support for prefilled slab sheafs

Add the prefilled sheaf structs to the slab header and the associated
functions to the testing/shared/linux.c file.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>