add test for regression with shutdown_timeout 0

[kykosic]

Motivation

Solves issue #3193

Added a regression test to make sure that Runtime::shutdown_timeout with Duration::from_nanos(0) instantly shuts down instead of waiting forever for all tasks.

Solution

Spawn a task that sleeps for 10k seconds, then time the call to shutdown_timeout with a zero duration. Assert that actual time to shutdown should be <1 second.

[Darksonn]

Thanks!