tests: kernel/userspace: skip other thread tests if needed

With memory domain enabled, all threads within the same domain
have access to each other threads' stacks, especially with
CONFIG_ARCH_MEM_DOMAIN_SYNCHRONOUS_API enabled (as it is
expected behavior). So update the conditions to skip both
tests to read and write to other threads' stacks.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>