Make debugger unloading deferred operation

test/cctest/test-debug.cc

Index: test/cctest/test-debug.cc
diff --git a/test/cctest/test-debug.cc b/test/cctest/test-debug.cc
index 918c7eccc31b26412935a8b2bac50e51dffb9bfb..21ba7ddb005cb6a809adc6d9efd3fcb92c00967d 100644
--- a/test/cctest/test-debug.cc
+++ b/test/cctest/test-debug.cc
@@ -443,6 +443,9 @@ void CheckDebuggerUnloaded(bool check_functions) {
 
 // Check that the debugger has been fully unloaded.
 static void CheckDebuggerUnloaded(bool check_functions = false) {
+  // Let debugger to unload itself synchronously
+  v8::Debug::ProcessDebugMessages();
+
   v8::internal::CheckDebuggerUnloaded(check_functions);
 }