tests: kernel: Add test to verify k_thread_user_mode_enter()
tests: kernel: Add test to verify k_thread_user_mode_enter()

Test to verify k_thread_user_mode_enter() when usermode is
not enabled or supported by architecture. The thread which calls
k_thread_user_mode_enter() with CONFIG_USERSPACE disabled or
architecture doesn't support usermode, should be marked
as usermode and if it is essential, then it has to be cleared.
This is added to improve code coverage.

Signed-off-by: Spoorthi K <c4ff46ff4cba9c2c6c6c9d55497ae534451b4309@intel.com>

explora26/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr