Fixed some cctest flakiness on mac.

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 436084af3724122e9727b3358b91bd7d8e287ae0..e724375b38462c2ec92c42f2d96e023593f87601 100644
--- a/test/cctest/test-debug.cc
+++ b/test/cctest/test-debug.cc
@@ -4522,6 +4522,7 @@ TEST(DebuggerAgent) {
   // with the client connected.
   ok = i::Debugger::StartAgent("test", kPort2);
   CHECK(ok);
+  i::Debugger::WaitForAgent();
   i::Socket* client = i::OS::CreateSocket();
   ok = client->Connect("localhost", port2_str);
   CHECK(ok);