[FIX] runtime: don't emit async hook warnings when cancelled/destroyed

[sdegueldre]

Previously, the async hook warning that's emitted when async hooks take
too long to resolve would be emitted even if the component was
destroyed or cancelled. We used to check if the node's fiber was still
the current fiber as a proxy for cancellation, but this is insufficient
in the case where an app might be destroyed.

This commit also wraps the content of some of the warning tests into a
try/finally block so that a failure of these tests doesn't pollute
setTimeout and console.warn and cause other tests to timeout one after
another.