ssh: use a variable rather than type for NoAuthError

Change-Id: Ib61e0bc7d953cefde0436f77fe6a610201043c85
Reviewed-on: https://go-review.googlesource.com/96336
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Spencer Tung <spencertung@google.com>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/client_auth_test.go

@@ -614,8 +614,8 @@ func TestClientAuthErrorList(t *testing.T) {
 	for i, e := range authErrs.Errors {
 		switch i {
 		case 0:
-			if _, ok := e.(*NoAuthError); !ok {
-				t.Fatalf("errors: got error type %T, want NoAuthError", e)
+			if e != NoAuthError {
+				t.Fatalf("errors: got error %v, want NoAuthError", e)
 			}
 		case 1:
 			if e != publicKeyErr {