ssh/agent: use right message to unlock agent, with related integration tests

Change-Id: Ie2e7618be63179fb65b8eea60684254712149a77
Reviewed-on: https://go-review.googlesource.com/50810
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/agent/server_test.go

@@ -43,7 +43,7 @@ func TestSetupForwardAgent(t *testing.T) {
 	defer a.Close()
 	defer b.Close()
 
-	_, socket, cleanup := startAgent(t)
+	_, socket, cleanup := startOpenSSHAgent(t)
 	defer cleanup()
 
 	serverConf := ssh.ServerConfig{