tests: add scrypt tests and move Go files from testdata

tests/valid_characters.go

@@ -0,0 +1,21 @@
+// 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.ArgsLine("!\"#$%&'", "()*+,-./", "01234567", "89:;<=>?", "@ABCDEFG",
+		"HIJKLMNO", "PQRSTUVW", "XYZ[\\]^_", "`abcdefg", "hijklmno", "pqrstuvw", "xyz{|}~")
+	f.Body([]byte(""))
+	f.X25519(testkit.TestX25519Recipient)
+	f.HMAC()
+	f.Payload("age")
+	f.Generate()
+}