test/common/test_fair_mutex: s/std::thread/std::async()/

test/common/test_fair_mutex: s/std::thread/std::async()/

for better readability, as we don't need to care about the details of
the executor. and the returned futures are blocked in their dtor as

> these actions will not block for the shared state to become ready,
> except that it may block if all of the following are true: the
> shared state was created by a call to std::async, the shared state
> is not yet ready, and this was the last reference to the shared state.

Signed-off-by: Kefu Chai <kchai@redhat.com>