[libc++] Fix wait_on_destruct.pass.cpp hanging sometimes (#146240)

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.

libcxx/test/std/thread/futures/futures.async/wait_on_destruct.pass.cpp

@@ -20,22 +20,24 @@
 #include <atomic>
 #include <future>
 #include <mutex>
+#include <condition_variable>
+#include <thread>
+
 
 std::mutex mux;
 
 int main(int, char**) {
-  using namespace std::chrono_literals;
+  std::condition_variable cond;
   std::unique_lock lock(mux);
-  std::atomic<bool> in_async = false;
-  auto v                     = std::async(std::launch::async, [&in_async, value = 1]() mutable {
-    in_async = true;
-    in_async.notify_all();
-    std::scoped_lock thread_lock(mux);
+  auto v = std::async(std::launch::async, [&cond, value = 1]() mutable {
+    std::unique_lock thread_lock(mux);
+    cond.notify_all();
+    thread_lock.unlock();
+
     value = 4;
     (void)value;
   });
-  in_async.wait(true);
-  lock.unlock();
+  cond.wait(lock);
 
   return 0;
 }