Improve guest function cancellation tests

[danbugs]

The guest cancellation tests call a function spin in either the host (

hyperlight/src/hyperlight_host/src/func/guest_dispatch.rs

Line 472 in b9c67fb

let result = ctx.call("CallHostSpin", ReturnType::Void, None);

) or the guest (

hyperlight/src/hyperlight_host/src/func/guest_dispatch.rs

Line 400 in b9c67fb

let result = ctx.call("Spin", ReturnType::Void, None);

), this means that the tests are probably never trying to terminate execution in the host control loop (e.g., calling a host function or dealing with some other VM exit).

We should improve the tests so that:
(1) we run many tests with variable cancellation time, and
(2) the function that we call in the guest should loop calling a function in the host which sleeps for a random amount of time.

This should ensure that we are testing cancellation across all states.