[test] Add missing tests and error handling to mechanism_test

I was looking around to get how vpnweb was working and I thought this would be a good "issue" to learn more about go and vpnweb.

To add the missing tests to mechanism_test, I refactor the code a little bit so GetAuthenticator is now returning an error instaed of using log.Fatal

Since doAuthenticationChecks was only used by sip2.GetAuthenticator I move it to the sip2.auth module.