Fix line wraps in test vector generator

Off by 2 + add slashes, similar to:

 The same example decoded for readability:

   =============== NOTE: '\\' line wrapping per RFC 8792 ===============

   {
     "header": { "alg": "CRYDI3", "kid": "did:example:123#key-0" },
     "payload": "It's a dangerous business, Frodo, going out your door.\
   \ You step onto the road, and if you don't keep your feet, there's\
   \ no knowing where you might be swept off to.",
     "signature": "2As8T1AHenWzLuTojcAYFDnT05n4bmDGIWenHqoXVizL7311HtVg\
   \7PEJHYmpc1fIvFNrm0xJt0asD5bQk3ZY8WuEQDUjsn4j+zbyob8MPQI5u3p5ZkqlLhG\
   \6Q8p1q0Hd5voY4a78vNxFJpYsETc0bECAft196z5hml2VjuDBqI7W4ju/iDKambJIDz\

mesur-io / post-quantum-signatures

Fix line wraps in test vector generator #65