add custom target `test-contrib` and make `make test` runs that
add custom target `test-contrib` and make `make test` runs that

picrin-scheme/picrin,omasanori/picrin,ktakashi/picrin,picrin-scheme/picrin,koba-e964/picrin,omasanori/picrin,dcurrie/picrin,koba-e964/picrin,leavesbnw/picrin,leavesbnw/picrin,ktakashi/picrin,dcurrie/picrin,koba-e964/picrin,ktakashi/picrin,leavesbnw/picrin