net/flowtrack: fix, test String method

I meant to do this in the earlier change and had a git fail.

To atone, add a test too while I'm here.

Updates #12486
Updates #12507

Change-Id: I4943b454a2530cb5047636f37136aa2898d2ffc7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/flowtrack/flowtrack.go

@@ -56,7 +56,9 @@ func (t Tuple) SrcPort() uint16 { return t.srcPort }
 func (t Tuple) DstPort() uint16 { return t.dstPort }
 
 func (t Tuple) String() string {
-	return fmt.Sprintf("(%v %v => %v)", t.proto, t.src, t.dst)
+	return fmt.Sprintf("(%v %v => %v)", t.proto,
+		netip.AddrPortFrom(t.SrcAddr(), t.srcPort),
+		netip.AddrPortFrom(t.DstAddr(), t.dstPort))
 }
 
 func (t Tuple) MarshalJSON() ([]byte, error) {

net/flowtrack/flowtrack_test.go

@@ -100,10 +100,15 @@ func BenchmarkMapKeys(b *testing.B) {
 	})
 }
 
-func TestJSON(t *testing.T) {
+func TestStringJSON(t *testing.T) {
 	v := MakeTuple(123,
 		netip.MustParseAddrPort("1.2.3.4:5"),
 		netip.MustParseAddrPort("6.7.8.9:10"))
+
+	if got, want := v.String(), "(IPProto-123 1.2.3.4:5 => 6.7.8.9:10)"; got != want {
+		t.Errorf("String = %q; want %q", got, want)
+	}
+
 	got, err := json.Marshal(v)
 	if err != nil {
 		t.Fatal(err)