net: change t.Error to t.Errorf

Caught by vet.

Updates #11041

Change-Id: I4dbb2eeaf633eea5976074840064edc2349e01d8
Reviewed-on: https://go-review.googlesource.com/27120
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/net/dnsmsg_test.go

@@ -393,7 +393,7 @@ func TestIsResponseTo(t *testing.T) {
 
 	for i := range badResponses {
 		if badResponses[i].IsResponseTo(&query) {
-			t.Error("%v: got true, want false", i)
+			t.Errorf("%v: got true, want false", i)
 		}
 	}
 }