https://crrev.com/c/3977111 factored out the integration tests for all
irqchip implementations, except for whpx.
Factor out the tests. Add `#[allow(unused)]` attribute for certain
helper functions that are not used in whpx. Make `interrupt_requested`
and `get_external_interrupt` public methods for tests to call into.
Test: Verified tests on windows. Using following command:
Test: `cargo test --tests -p devices --features="whpx,slirp" whpx -- --nocapture --test-threads 1`
Change-Id: I1863d509357193fdbc309e90cd0631fe5849a3bc
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3993814
Commit-Queue: Vaibhav Nagarnaik <vnagarnaik@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
