tests: gen_isr_table: move multilevel test into a separate file

The multilevel IRQ APIs tests are currently cordoned off by
`CONFIG_MULTI_LEVEL_INTERRUPTS` compiler switch, do this more
cleanly by moving it to another file and use cmake for that
instead.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>