tests: userspace: fix tests that don't hold on MMU
tests: userspace: fix tests that don't hold on MMU

MMU threads within the same memory domain have access to
each other's stacks.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr