Bug
1861627 join MockCubeb thread from main thread r=padenot
The main thread is available later in shutdown than the background thread pool
[1] and leaks [2] the std::thread without crashing if no longer available for
the join().
The MockCubeb thread function is about to return so the main thread should not
need to wait long.
[1]
https://searchfox.org/mozilla-central/rev/
01a0d864a9442d0fe2dbd4beee5c88b9b46e96bd/xpcom/build/XPCOMInit.cpp#628,668
[2]
https://searchfox.org/mozilla-central/rev/
01a0d864a9442d0fe2dbd4beee5c88b9b46e96bd/xpcom/threads/nsThreadUtils.cpp#233
Differential Revision: https://phabricator.services.mozilla.com/
D192458