Add pk_write test cases where the ASN.1 INTEGER encoding of the
private value would not have the mandatory size for the OCTET STRING
that contains the value.
ec_256_long_prv.pem is a random secp256r1 private key, selected so
that the private value is >= 2^255, i.e. the top bit of the first byte
is set (which would cause the INTEGER encoding to have an extra
leading 0 byte).
ec_521_short_prv.pem is a random secp521r1 private key, selected so
that the private value is < 2^519, i.e. the first byte is 0 and the
top bit of the second byte is 0 (which would cause the INTEGER
encoding to have one less 0 byte at the start).
