REGTEST: make the "run-regtests" script search for tests in reg-tests by default

It happens almost daily to me that make regtests fails because the script
found a temporary, old, or broken VTC file that was lying in my work dir,
leaving me no place to hide it. This is a real pain as some tests take ages
to fail, so let's make this script only look up for tests where they are
expected to be stored, under reg-tests only. It remains possible to force
the location on the command line though.

scripts/run-regtests.sh

@@ -332,7 +332,7 @@ if [ $(_version "$HAPROXY_VERSION") -lt $(_version "1.9") ]; then
 fi
 
 if [ -z "$REGTESTS" ]; then
-  _findtests ./
+  _findtests reg-tests/
 else
   for t in $REGTESTS; do
     _findtests $t