chore(tests): use on_finish to always keep appveyor artifacts

after_tests only runs if the tests succeed. If there is an error then we need to use on_finish per
http://help.appveyor.com/discussions/problems/3543-cannot-save-test-artifacts-when-tests-fail

Author: jimthedev

appveyor.yml

@@ -17,13 +17,12 @@ test_script:
   # run tests
   - npm test
 
-after_test:
+on_finish:
   - ps: $root = Resolve-Path .\test\artifacts; [IO.Directory]::GetFiles($root.Path, '*.*', 'AllDirectories') | % { Push-AppveyorArtifact $_ -FileName $_.Substring($root.Path.Length + 1) -DeploymentName to-publish }
 
 artifacts:
   - path: test/artifacts/**/*
     name: test artifacts
 
 # Don't actually build.
-build: off
-  
+build: off