Wait testing servers to start before running tests

Nit: like with httpServer.py and echoServer.py, this doesn't need to `cd tests` before it starts waitServers.py. Only httpsServer.py and sslEchoServer.py need to start with the working directory set to tests/, because they reference the cert.pem file by relative path. Doing it for waitServers.py doesn't cause any harm, however.