irq: Add KUnit test for refcounted interrupt enable/disable

While making changes to the refcounted interrupt patch series, at some
point on my local branch I broke something and ended up writing some kunit
tests for testing refcounted interrupts as a result. So, let's include
these tests now that we have refcounted interrupts.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Link: https://patch.msgid.link/20260121223933.1568682-7-lyude@redhat.com