tests: add more tests for hmac and X25519 encodings

tests/hmac_missing.go

@@ -0,0 +1,19 @@
+// Copyright 2022 The age Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+package main
+
+import "filippo.io/age/internal/testkit"
+
+func main() {
+	f := testkit.NewTestFile()
+	f.VersionLine("v1")
+	f.X25519(testkit.TestX25519Identity)
+	f.HMACLine(nil)
+	f.Payload("age")
+	f.ExpectHeaderFailure()
+	f.Generate()
+}