In order for josepy.test_util._guess_loader to work properly, make sure
to use appropriate extension for vector filenames: .pem for PEM and
.der for DER.

The following command has been used to generate test keys:

  for x in 256 512 1024 2048; do openssl genrsa -out rsa${k}_key.pem $k; done

and for the CSR:

  openssl req -key rsa2048_key.pem -new -subj '/CN=example.com' -outform DER > csr.der

and for the certificate:

  openssl req -key rsa2047_key.pem -new -subj '/CN=example.com' -x509 -outform DER > cert.der
