A key pair made only for these tests, with:

  openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out test-private-key.pem
  openssl pkey -in test-private-key.pem -pubout -out test-public-key.pem

It signs nothing real and is registered nowhere. openssl-rs256-signature.txt is
the RS256 signature openssl made of the text "header.payload", in the URL-safe
Base64 of JWTs:

  printf 'header.payload' | openssl dgst -sha256 -sign test-private-key.pem
