[libc++] Try to fix timing-sensitive future test

[ldionne]

Summary

This test has been failing spuriously in our CI for a long time. This
commit changes the timings used in the test in an attempt to make it
more resilient.

[EricWF]

Can you try simply marking the test with // FLAKY_TEST.?

If that doesn't work, this LGTM.

[ldionne]

In D75148#1894455, @EricWF wrote:

Can you try simply marking the test with // FLAKY_TEST.?

If that doesn't work, this LGTM.

Done.

libcxx/test/std/thread/futures/futures.unique_future/wait_for.pass.cpp

 
 template <typename T, typename F>
 void test(F func) {
     typedef std::chrono::high_resolution_clock Clock;
     std::promise<T> p;
     std::future<T> f = p.get_future();
     std::thread(func, std::move(p)).detach();
     assert(f.valid());
-    assert(f.wait_for(ms(300)) == std::future_status::timeout);
+    assert(f.wait_for(ms(50)) == std::future_status::timeout);
     assert(f.valid());
-    assert(f.wait_for(ms(300)) == std::future_status::ready);
+    assert(f.wait_for(ms(600)) == std::future_status::ready);
     assert(f.valid());
     Clock::time_point t0 = Clock::now();
     f.wait();
     Clock::time_point t1 = Clock::now();
     assert(f.valid());
     assert(t1-t0 < ms(50));
 }
 
 int main(int, char**)
 {
     test<int>(func1);
     test<int&>(func3);
     test<void>(func5);
     return 0;
 }