Let the second pass phantom callbacks run in a separate task on the foreground thread.

test/cctest/cctest.cc

Index: test/cctest/cctest.cc
diff --git a/test/cctest/cctest.cc b/test/cctest/cctest.cc
index b5771ff6550681526cda8cd6a9a3c36411645482..35e7665cde46ee359f082a48318a70b1925f3e21 100644
--- a/test/cctest/cctest.cc
+++ b/test/cctest/cctest.cc
@@ -97,6 +97,7 @@ void CcTest::Run() {
   }
   callback_();
   if (initialize_) {
+    EmptyMessageQueues(isolate_);
     isolate_->Exit();
   }
 }