This test was deadlocking on my machine.
It seems to me the intention of `in_async.wait(...)` was to wait for the
value to be set to true, which requires a call of `wait(false)` (waits
if value matches argument).
~As a drive by change scoped_lock to unique_lock, since there shouldn't
be any functional difference between the two in this test.~
I've addressed the issues with the `in_async` by switching to a
condition variable instead, since my first attempt at fixing this with
`in_async` wasn't sufficient.
4aaab69314