quinn: `endpoint_wait_idle` failing test

[ramfox]

I found what I think is a bug in quinn, and this PR adds a test illustrating the bug.

The endpoint_wait_idle test shows that if an endpoint attempts to connect to an address, but the connection never gets beyond the Connecting stage, it causes the wait_idle call (when trying to gracefully shutdown the endpoint) to always take around 3s to return.

I ran into this personally when a call to Endpoint::connect was made in a task that was aborted early. Suddenly, calls to wait_idle were taking around 3s, when they were previously <200ms.

I'm hoping to get insight on to what could be causing this, so that either I can take a stab at fixing, or someone with more quinn experience can.