[libcxx] [test] Fix spurious failures in the thread join test on Windows

Make sure that the detached thread has started up before exiting
the process.

This is exactly the same fix as D105592, with the same pattern
being present in a different test case.

Differential Revision: https://reviews.llvm.org/D105736

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp

@@ -19,10 +19,13 @@
 #include <cstdlib>
 #include <cassert>
 #include <system_error>
+#include <atomic>
 
 #include "make_test_thread.h"
 #include "test_macros.h"
 
+std::atomic_bool done(false);
+
 class G
 {
     int alive_;
@@ -45,7 +48,7 @@ class G
 int G::n_alive = 0;
 bool G::op_run = false;
 
-void foo() {}
+void foo() { done = true; }
 
 int main(int, char**)
 {
@@ -72,6 +75,11 @@ int main(int, char**)
             assert(false);
         } catch (std::system_error const&) {
         }
+        // Wait to make sure that the detached thread has started up.
+        // Without this, we could exit main and start destructing global
+        // resources that are needed when the thread starts up, while the
+        // detached thread would start up only later.
+        while (!done) {}
     }
 #endif