For those that face this…

You have to update two things.

Add the client key and cert to the config in config/files.go

var (
	CAFile         = configFile("ca.pem")
	ServerCertFile = configFile("server.pem")
	ServerKeyFile  = configFile("server-key.pem")
	ClientCertFile = configFile("client.pem")               // <---
	ClientKeyFile  = configFile(("client-key.pem"))    // <---
)

Update the client tls configuration to utilize the generated client cert and key files added to the config in server/server_test.go

	clientTLSConfig, err := config.SetupTLSConfig(config.TLSConfig{
		CertFile: config.ClientCertFile,              // <---
		KeyFile:  config.ClientKeyFile,               // <---
		CAFile:   config.CAFile,
		Server:   false,                                          // <---
	})