Add a simple test case that creates an enveloped-data structure without
using the shorthand method, and fix two issues preventing this from
working correctly.
First, OpenSSL::PKey::PKCS7#add_recipient currently inserts an
incomplete PKCS7_RECIP_INFO object into the PKCS7 object. When
duplicating an unfinalized PKCS7_RECIP_INFO, the internal X509 reference
must also be copied, as it is later used by #add_data to fill the rest.
A similar issue with #add_signer was fixed in commit 20ca7a27a8
(pkcs7: keep private key when duplicating PKCS7_SIGNER_INFO,
2021-03-24).
Second, #add_data calls PKCS7_dataFinal(), which for enveloped-data
appears to require the BIO to be flushed explicitly with BIO_flush().
Without this, the last block of the encrypted data would be missing.
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