test: fix flaky ClientMultithreadTest

`ClientMultithreadTest` fails on MacOS because the `connect`s are timing
out. To fix this, let's increase the `connect_timeout` for
`ClientMultithreadTest` to 10 seconds.

Closes #150

Author: CuriousGeorgiy

test/ClientMultithreadTest.cpp

@@ -47,6 +47,7 @@ test_connect(Connector &client, Connection &conn, const std::string &addr, unsig
 				      .address = addr,
 				      .service = service,
 				      .transport = transport,
+				      .connect_timeout = 10,
 				      .user = user,
 				      .passwd = passwd,
 			      });