Fix ASN1 Integer & Timestamp encoding and add test files

[JRomainG]

While encoding an x509 certificate to protobuf and back, I encountered some issues parsing the certificate generated through x509_certificate::X509CertificateToDER.

I address 2 issues I found and add the test files I used in a dedicated directory. I still haven't been able to properly convert extensions from protobuf to DER, but since they are optional this could wait another pull request.

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