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{} }