Move back to using subst for testing

This was originally removed in sass#1514 to work around nodejs/node#6500
which has since been resolved. This isn't playing nice with our npm
cache in CI because we're changing the current path after running
npm install.

appveyor.yml

@@ -122,6 +122,4 @@
     - git submodule update --init --recursive
     - npm install --msvs_version=2013
 
-  test_script:
-    - ps: set-location -path c:\projects\node_modules\node-sass
-    - npm test
+  test_script: npm test