REGTESTS: use lua mailer script for mailers tests

Since mailers/healthcheckmail.vtc already requires lua to emulate the
SMTP server for the test, force it to use lua mailers example script
to send email-alerts so we don't rely anymore on legacy tcpcheck
mailers implementation.

This is done by simply loading examples/mailers.lua (as a symlink) from
haproxy config file.

reg-tests/mailers/healthcheckmail.vtc

@@ -12,6 +12,7 @@ syslog S1 -level notice {
 
 haproxy h1 -conf {
     global
+        lua-load ${testdir}/mailers.lua
         lua-load ${testdir}/healthcheckmail.lua
 
     defaults

reg-tests/mailers/mailers.lua

@@ -0,0 +1 @@
+../../examples/lua/mailers.lua