tests: interrupt: refactor the sw_isr_irq_parent_table tests

Split the `sw_isr_irq_parent_table` test into one that tests
the IRQ table index function which is generic and should
always be tested, and one that tests multi-instance -related
functions which only work in interrupt controller drivers that
has multi-instance implementation.

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