BUILD: Makefile: move REGTESTST_TYPE default setting

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.