run/runtest.sh: strip '@' from start of argument

a little convenience function: when you forget to delete '@' from scenario tag supplied as argument to run/runtest.sh, it will strip it for you and use just the rest of argument: "run/runtest.sh @pass" will now be the same as "run/runtest.sh pass". It's however no smarter than that so "run/runtest.sh @@pass" would just become "run/runtest.sh @pass"