net: fix nits found by go tool vet

R=golang-dev, dave, adg
CC=golang-dev
https://golang.org/cl/27430043

src/pkg/net/net_test.go

@@ -231,12 +231,12 @@ func TestErrorNil(t *testing.T) {
 	// Make Listen fail by relistening on the same address.
 	l, err := Listen("tcp", "127.0.0.1:0")
 	if err != nil {
-		t.Fatal("Listen 127.0.0.1:0: %v", err)
+		t.Fatalf("Listen 127.0.0.1:0: %v", err)
 	}
 	defer l.Close()
 	l1, err := Listen("tcp", l.Addr().String())
 	if err == nil {
-		t.Fatal("second Listen %v: %v", l.Addr(), err)
+		t.Fatalf("second Listen %v: %v", l.Addr(), err)
 	}
 	if l1 != nil {
 		t.Fatalf("Listen returned non-nil interface %T(%v) with err != nil", l1, l1)
@@ -245,12 +245,12 @@ func TestErrorNil(t *testing.T) {
 	// Make ListenPacket fail by relistening on the same address.
 	lp, err := ListenPacket("udp", "127.0.0.1:0")
 	if err != nil {
-		t.Fatal("Listen 127.0.0.1:0: %v", err)
+		t.Fatalf("Listen 127.0.0.1:0: %v", err)
 	}
 	defer lp.Close()
 	lp1, err := ListenPacket("udp", lp.LocalAddr().String())
 	if err == nil {
-		t.Fatal("second Listen %v: %v", lp.LocalAddr(), err)
+		t.Fatalf("second Listen %v: %v", lp.LocalAddr(), err)
 	}
 	if lp1 != nil {
 		t.Fatalf("ListenPacket returned non-nil interface %T(%v) with err != nil", lp1, lp1)