Function check_wait_until

tests/BS_thread_pool_test.cpp:1515–1535

* @brief Check that wait_until() works. */

 * @brief Check that wait_until() works.
 */
void check_wait_until()
{
    constexpr std::chrono::milliseconds long_sleep_time(250);
    constexpr std::chrono::milliseconds short_sleep_time(10);
    BS::thread_pool pool;
    logln("Checking that wait_until() works...");
    std::atomic<bool> done = false;
    pool.detach_task(
        [&done, long_sleep_time]
        {
            std::this_thread::sleep_for(long_sleep_time);
            done = true;
        });
    const std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
    logln("Task that lasts ", long_sleep_time.count(), "ms submitted. Waiting until ", short_sleep_time.count(), "ms from submission time...");
    pool.wait_until(now + short_sleep_time);
    check(!done);
    logln("Waiting until ", long_sleep_time.count() * 2, "ms from submission time...");
    pool.wait_until(now + long_sleep_time * 2);
    check(done);
}

// An auxiliary thread pool used by check_wait_multiple_deadlock(). It's a global variable so that the program will not get stuck upon destruction of this pool if a deadlock actually occurs.
BS::thread_pool check_wait_multiple_deadlock_pool;