static-test: use base reference as base branch instead of master
static-test: use base reference as base branch instead of master

This way they will also work with release branches ;-).

authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT