Fix API test cases

- Add meaningful app->argv/c
- Add support for subfolder install
- Add meaningful image URL from placeholder.com
- Fix incomplete test testApiHelpTestWithXml()
- Use $this->frienduser for direct messages tests
- Fix dataset to have an actual contact relationship

tests/datasets/api.yml

@@ -34,7 +34,7 @@ contact:
         network: dfrn
     -
         id: 43
-        uid: 0
+        uid: 42
         # Having the same name and nick allows us to test
         # the fallback to api_get_nick() in api_get_user()
         name: othercontact