Revive regress target and kill off test target instead

bin/test being present causes the unfortunate issue of make test being run on
the bin/ subdir, which subsequently causes things to fall apart when running
make buildworld

make regress is the legacy target, so use it instead

While here, in order to deal with recursion issues and executing make regress
both at subdirectories (which could yield the wrong results depending on how
things are installed)