Construct SCT from base64 in ct_test

This gives better code coverage and is more representative of how a
user would likely construct an SCT (using the base64 returned by a CT log).

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1548)