make yarn test-unit command more cross-platform (mapbox#7528)

Author: booo

build/run-tap

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 if [ "$#" == 5 ]; then
-  arg="${@:0:4} ${BASH_ARGV[1]}/${BASH_ARGV[0]}"
+  arg="${@:1:3} ${4}/${5}"
 else
   arg="${@}"
 fi