x509_acert: Add simple API tests

Add a some simple API tests for reading, printing, signing
and verifying attribute certificates.

Reviewed-by: Tomas Mraz <tomas@openssl.org>

x509_acert: Add simple API tests

Add a some simple API tests for reading, printing, signing
and verifying attribute certificates.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15857)

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