Potential thread leak in rt_common tests

[tiif]

Version
tokio master branch dcae2b9e

Description
When running with cargo miri test --features full --test rt_common, Miri reported error: the main thread terminated without waiting for all remaining threads on the rt_common tests below:

tokio::rt_common current_thread_scheduler::shutdown_timeout_0
tokio::rt_common current_thread_scheduler::shutdown_timeout
tokio::rt_common threaded_scheduler_1_thread::shutdown_timeout
tokio::rt_common threaded_scheduler_1_thread::shutdown_timeout_0
tokio::rt_common threaded_scheduler_4_threads::shutdown_timeout_0
tokio::rt_common threaded_scheduler_4_threads::shutdown_timeout

To reproduce, #![cfg(not(miri))] needs to be removed.

(I am not sure if this is an intended effect, but I will just file it under C-bug just in case this is something that should be fixed.)

[Darksonn]

Plausibly we could make the background threads exit more quickly, but it's not possible to actually join them. The shutdown_timeout API explicitly does not join the threads when the timeout hits, after all.

[tiif]

This probably falls under rust-lang/miri#1371. Since it is working as expected, I will just close this issue, thanks for answering!