Fix the krb5 external test

The krb5 external test relies on legacy algorithms. Therefore we make
use of the capability to load a config file by default, and ensure that
the config file in use by the krb5 tests loads both the legacy and default
providers.

[extended tests]

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9492)