BUILD: Makefile: move REGTESTST_TYPE default setting

In patch 3bad3d5 ("BUILD: Makefile: exclude broken tests by default"),
the default setting of the REGTESTST_TYPE variable was set in the
Makefile instead of the run-regtests.sh script.

Doing it in the Makefile was breaking the use of this environment
varible with make ( REGTESTS_TYPES=slow,default make reg-tests )

This patch move the default setting from the Makefile to
run-regtests.sh. It also change the documentation in `make
reg-tests-help` about the default value.

This patch should be backported where 3bad3d5 is backported.

(cherry picked from commit c1ddcafdf9b425ff4ac2f1e3f1662a9133fc4cf6)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/scripts/run-regtests.sh b/scripts/run-regtests.sh
index 27bb13c..b3e4b13 100755
--- a/scripts/run-regtests.sh
+++ b/scripts/run-regtests.sh
@@ -128,7 +128,7 @@
 _findtests() {
   set -f
 
-  REGTESTS_TYPES="${REGTESTS_TYPES:-any}"
+  REGTESTS_TYPES="${REGTESTS_TYPES:-default,bug,devel,slow}"
   any_test=$(echo $REGTESTS_TYPES | grep -cw "any")
   for i in $( find "$1" -name *.vtc ); do
     skiptest=