TEST: Add new serializer and deserializer test

TEST: Add new serializer and deserializer test

This test revolves around a central function that will first serialize
an EVP_PKEY, then deserialize the result into a new EVP_PKEY and
compare the two.

The following tests are currently implemented:

1.  EVP_PKEY (RSA) -> DER, then DER -> EVP_PKEY (RSA).
2.  EVP_PKEY (RSA) -> PEM, then PEM -> EVP_PKEY (RSA).
    This one exercises deserializer chains, as we know that there is a
    PEM -> DER and a DER -> EVP_PKEY (RSA) deserializer, but no direct
    PEM -> EVP_PKEY (RSA) deserializer.

Additionally, a small fix in test_fail_string_common(), as strcmp()
could run past a buffer if one of the strings isn't terminated with
a null byte within the given length.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12410)