ztest: use ASSERT3P to compare pointers

With a sufficiently modern gcc (I saw this with gcc13), gcc complains
when casting pointers to an integer of a different type (even a larger
one). On 32-bt ASSERT3U does this on 32-bit systems by casting a 32-bit
pointer to uint64_t so use ASSERT3P which uses uintptr_t.

Fixes: 5caeef02fa53 RAID-Z expansion feature

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #16115