(workaround for ci, to be dropped after bitcoin-core#214 is merged)

Cherry-picking 73d22ba from bitcoin-core#222

Author: ryanofsky

test/mp/test/test.cpp

@@ -321,12 +321,12 @@ KJ_TEST("Make simultaneous IPC callbacks with same request_thread and callback_t
     setup.server->m_impl->m_fn = [&] {};
     foo->callFnAsync();
     ThreadContext& tc{g_thread_context};
-    std::optional<Thread::Client> callback_thread, request_thread;
-    {
+    Thread::Client *callback_thread, *request_thread;
+    foo->m_context.loop->sync([&] {
         Lock lock(tc.waiter->m_mutex);
-        callback_thread = tc.callback_threads.at(foo->m_context.connection)->m_client;
-        request_thread = tc.request_threads.at(foo->m_context.connection)->m_client;
-    }
+        callback_thread = &tc.callback_threads.at(foo->m_context.connection)->m_client;
+        request_thread = &tc.request_threads.at(foo->m_context.connection)->m_client;
+    });
 
     setup.server->m_impl->m_fn = [&] {
         try