ssh: disable client agent tests on Windows

ssh-agent is implemented as a Windows service and exposed on a
named pipe. We don't currently support it.

See golang/go#60981

Change-Id: Iebdc42db30b37a87ac0766231b16aff3f17b3f56
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/509035
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/agent/client_test.go

@@ -29,6 +29,9 @@ func startOpenSSHAgent(t *testing.T) (client ExtendedAgent, socket string, clean
 		// types supported vary by platform.
 		t.Skip("skipping test due to -short")
 	}
+	if runtime.GOOS == "windows" {
+		t.Skip("skipping on windows, we don't support connecting to the ssh-agent via a named pipe")
+	}
 
 	bin, err := exec.LookPath("ssh-agent")
 	if err != nil {