REGEST: Add reg tests about error files

REGEST: Add reg tests about error files

2 reg tests are added. The first one ensures the declaration of errors in a
proxy is fonctionnal. It declares http-errors sections and declare error files
using the errorfile and the errorfiles directives, both in the default section
and the frontend sections. The second one ensures it is possible to use a custom
error file for an HTTP deny rule.