Don't use subst for testing

Node 6.x has trouble including modules
from MS-DOS drives created with subst.exe.

Since we use use subst only to set
symbols paths in the PDB file, let's
revert to the C: drive when doing tests.

Workaround for: nodejs/node#6500

appveyor.yml

@@ -50,7 +50,9 @@
     - git submodule update --init --recursive
     - npm install --msvs_version=2013
 
-  test_script: npm test
+  test_script:
+    - ps: set-location -path c:\projects\node_modules\node-sass
+    - npm test
 
   before_deploy:
     # Save artifacts with full qualified names of binding.node and binding.pdb
@@ -113,4 +115,6 @@
     - git submodule update --init --recursive
     - npm install --msvs_version=2013
 
-  test_script: npm test
+  test_script:
+    - ps: set-location -path c:\projects\node_modules\node-sass
+    - npm test