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.
diff --git a/Makefile b/Makefile
index b0ab6bc..81d9b3b 100644
--- a/Makefile
+++ b/Makefile
@@ -1044,7 +1044,6 @@
 	@echo 'OBJS="$(strip $(OBJS))"'
 
 ifeq (reg-tests, $(firstword $(MAKECMDGOALS)))
-  REGTESTS_TYPES := default,bug,devel,slow
   REGTEST_ARGS := $(wordlist 2, $(words $(MAKECMDGOALS)), $(MAKECMDGOALS))
   $(eval $(REGTEST_ARGS):;@true)
 endif
@@ -1074,7 +1073,7 @@
 	@echo "To run tests with specific types:"
 	@echo "    $$ REGTESTS_TYPES=slow,default make reg-tests"
 	@echo
-	@echo "with 'any' as default value for REGTESTS_TYPES variable."
+	@echo "with 'default,bug,devel,slow' as default value for REGTESTS_TYPES variable."
 	@echo
 	@echo "About the reg test types:"
 	@echo "    any         : all the tests without distinction (this is the default"