Add test of busy waiting std::thread to start. NFC (#15896)

This change adds a test that demonstrates the issues with std::thread
and busy-waiting for them to start (on the main thread).

Fixing the issue is left as a followup.

See #15868

tests/pthread/test_pthread_busy_wait.cpp

@@ -0,0 +1,24 @@
+#include <atomic>
+#include <thread>
+#include <cstdio>
+#include <emscripten/threading.h>
+
+int main() {
+  std::atomic<bool> done(false);
+
+  std::thread t([&]{
+    printf("in thread\n");
+    done = true;
+  });
+
+  while (!done) {
+    // TODO(sbc): this call should not be needed.
+    // See: https://github.com/emscripten-core/emscripten/issues/15868
+    emscripten_main_thread_process_queued_calls();
+    std::this_thread::yield();
+  }
+
+  t.join();
+  printf("done\n");
+  return 0;
+}

tests/pthread/test_pthread_busy_wait.out

@@ -0,0 +1,2 @@
+in thread
+done

tests/pthread/test_pthread_cxx_threads.cpp

@@ -1,6 +1,10 @@
 #include <thread>
 
 int main() {
-  std::thread([]{}).join();
+  std::thread t([]{
+    printf("in thread\n");
+  });
+  t.join();
+  printf("done\n");
   return 0;
 }

tests/pthread/test_pthread_cxx_threads.out

@@ -0,0 +1,2 @@
+in thread
+done

tests/test_core.py

@@ -8476,12 +8476,16 @@ def test_pthread_c11_threads(self):
 
   @node_pthreads
   def test_pthread_cxx_threads(self):
-    self.set_setting('PROXY_TO_PTHREAD')
-    self.clear_setting('ALLOW_MEMORY_GROWTH')
-    self.set_setting('INITIAL_MEMORY', '64Mb')
+    self.set_setting('PTHREAD_POOL_SIZE', 1)
     self.set_setting('EXIT_RUNTIME')
     self.do_run_in_out_file_test('pthread/test_pthread_cxx_threads.cpp')
 
+  @node_pthreads
+  def test_pthread_busy_wait(self):
+    self.set_setting('PTHREAD_POOL_SIZE', 1)
+    self.set_setting('EXIT_RUNTIME')
+    self.do_run_in_out_file_test('pthread/test_pthread_busy_wait.cpp')
+
   @node_pthreads
   def test_pthread_create_pool(self):
     # with a pool, we can synchronously depend on workers being available