Fix ct_test to not assume it's in the source directory

ct_test assumed it's run in the source directory and failed when built
elsewhere.  It still defaults to that, but can be told another story
with the environment variables CT_DIR and CERTS_DIR.

Test recipe updated to match.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>