tests: kernel: interrupt: Clean up dynamic_isr test
tests: kernel: interrupt: Clean up dynamic_isr test

This commit cleans up the dynamic ISR test and adds a documentation
comment for it.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

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