Add test vector for (invalid) mixed-case offer encoding (lightning#1217)

The spec requires the bech32 offer encodings to be all upper- or
lowercase, but disallows mixed-case encodings.

Here, we add a test vector for this.

bolt12/format-string-test.json

@@ -29,6 +29,11 @@
 	"valid": true,
 	"string": "LNO1PQPS7SJQPGT+ YZM3QV4UXZMTSD3JJQER9WD3HY6TSW3+  5K7MSJZFPY7NZ5YQCN+\nYGRFDEJ82UM5WF5K2UCKYYPWA3EYT44H6TXTXQUQH7LZ5DJGE4AFGFJN7K4RGRKUAG0JSD+\r\n 5XVXG"
     },
+    {
+	"comment": "Mixed case is invalid",
+	"valid": false,
+	"string": "LnO1PqPs7sJqPgTyZm3qV4UxZmTsD3JjQeR9Wd3hY6TsW35k7mSjZfPy7nZ5YqCnYgRfDeJ82uM5Wf5k2uCkYyPwA3EyT44h6tXtXqUqH7Lz5dJgE4AfGfJn7k4rGrKuAg0jSd5xVxG"
+    },
     {
 	"comment": "+ must be surrounded by bech32 characters",
 	"valid": false,