You should be able to make self-signed certificates, then configure the test server to accept those for test purposes. If you go this route, you might consider adding a testing flag to disable those certificates, in case they somehow accidentally get deployed.
However, WCF authentication is quite flexible, so it should not be hard to switch over to other forms, such a user/pass.
Finally, if you are using Active Directory, you can enable the AD Certificate Mapper, and attach certificates to user accounts. The application will see the authentication as Windows authentication -- the certificate part becomes transparent. This allows you to simple reconfigure AD, or switch to, say, NTLM, without reconfiguring your application.
If you post more info on your setup, you might get better answers.