Add IPv6 overlay address support to client interface and engine

client/iface/wgaddr/address_test_helpers.go

@@ -0,0 +1,10 @@
+package wgaddr
+
+// MustParseWGAddress parses and returns a WG Address, panicking on error.
+func MustParseWGAddress(address string) Address {
+	a, err := ParseWGAddress(address)
+	if err != nil {
+		panic(err)
+	}
+	return a
+}