tests: mem_protect: avoid allocating K_OBJ_MSGQ in userspace.

The K_OBJ_MSGQ object is unitialized so when the thread cleanup occurs
after an expected fault for invalid access the test case can randomly
fault again because the cleanup of the thread will sometimes attempt
to free invalid buffer_start pointer in the msgq object.

Fixes #42705

Signed-off-by: David Leach <david.leach@nxp.com>