tests: kernel: userspace: correct address of thread priv stack start

In ARM architecture z_priv_stack_find() returns the start of a
thread's privilege stack; we do not need to subtract the length
of a (possible) stack guard. This commit corrects the assigning
of the start address of a thread privilege stack in
test/kerne/mem_protect/mem_protect/userspace.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>