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.

(cherry picked from commit a5afb0bf3668851d130d75ccd7dbf4fcadd3a2b7)
[wt: needed since the backport of 60837d340 ("REGTEST: Add a simple
 script to tests errorfile directives in proxy sections")]
Signed-off-by: Willy Tarreau <w@1wt.eu>