Fix krb5 external test failure

The krb5 test requires the legacy module to be loaded in order to work.
It also seems to be senstive to using relative paths, so we use absolute
ones instead.

[extended tests]

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10992)