Improve test coverage and fix issue in path loading (#602)

* reactivate skipped tests

* remove unused internal interfaces

* enable pytest in VS Code

* simplify and fix path logic in loader

* increase test coverage

* test missing config

* check for wrong printer names

* Skipped appdir test

.vscode/settings.json

@@ -14,4 +14,6 @@
   "editor.codeActionsOnSave": {
     "source.organizeImports": true
   },
+  "python.testing.unittestEnabled": false,
+  "python.testing.pytestEnabled": true,
 }