Minor cleanups for staticcheck issues

staticcheck found a couple of minor code cleanup improvements, like
unused variables or an out-of-order defer, mostly in tests.

This patch fixes those problems by making the necessary adjustments.

They're all fairly small, and should not change the logic in any
significant way.

test/util/smtpc/smtpc.go

@@ -56,11 +56,11 @@ func main() {
 		}
 
 		conn, err = tls.Dial("tcp", *addr, tlsConfig)
-		defer conn.Close()
 	} else {
 		conn, err = net.Dial("tcp", *addr)
 	}
 	notnil(err)
+	defer conn.Close()
 
 	// Send the message.
 	client, err := smtp.NewClient(conn, *addr)