tailcfg: add marshaling round-tripping test.

Temporary until #3206 goes away, but having changed the marshal/unmarshal
implementation I got nervous about the new one doing the correct thing.
Thankfully, the test says it does.

Signed-off-by: David Anderson <danderson@tailscale.com>

tailcfg/tailcfg.go

@@ -1160,6 +1160,9 @@ func (k *NodeKey) UnmarshalText(text []byte) error {
 	nk.AppendTo(k[:0])
 	return nil
 }
+func (k NodeKey) AsNodePublic() key.NodePublic {
+	return key.NodePublicFromRaw32(mem.B(k[:]))
+}
 
 // IsZero reports whether k is the zero value.
 func (k NodeKey) IsZero() bool { return k == NodeKey{} }

tailcfg/tailcfg_test.go

@@ -5,6 +5,7 @@
 package tailcfg
 
 import (
+	"bytes"
 	"encoding"
 	"encoding/json"
 	"reflect"
@@ -415,6 +416,44 @@ func TestNodeKeyMarshal(t *testing.T) {
 	testKey(t, "nodekey:", k1, &k2)
 }
 
+func TestNodeKeyRoundTrip(t *testing.T) {
+	serialized := `{
+      "Pub":"nodekey:50d20b455ecf12bc453f83c2cfdb2a24925d06cf2598dcaa54e91af82ce9f765"
+    }`
+
+	// Carefully check that the expected serialized data decodes and
+	// re-encodes to the expected keys. These types are serialized to
+	// disk all over the place and need to be stable.
+	pub := NodeKey{
+		0x50, 0xd2, 0xb, 0x45, 0x5e, 0xcf, 0x12, 0xbc, 0x45, 0x3f, 0x83,
+		0xc2, 0xcf, 0xdb, 0x2a, 0x24, 0x92, 0x5d, 0x6, 0xcf, 0x25, 0x98,
+		0xdc, 0xaa, 0x54, 0xe9, 0x1a, 0xf8, 0x2c, 0xe9, 0xf7, 0x65,
+	}
+
+	type key struct {
+		Pub NodeKey
+	}
+
+	var a key
+	if err := json.Unmarshal([]byte(serialized), &a); err != nil {
+		t.Fatal(err)
+	}
+	if a.Pub != pub {
+		t.Errorf("wrong deserialization of public key, got %#v want %#v", a.Pub, pub)
+	}
+
+	bs, err := json.MarshalIndent(a, "", "  ")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var b bytes.Buffer
+	json.Indent(&b, []byte(serialized), "", "  ")
+	if got, want := string(bs), b.String(); got != want {
+		t.Error("json serialization doesn't roundtrip")
+	}
+}
+
 func TestDiscoKeyMarshal(t *testing.T) {
 	var k1, k2 DiscoKey
 	for i := range k1 {