Path of group_vars and host_vars were getting the basedir added twice.

Fix inventory so this won't happen and fix DataLoader so that it will
test relative paths relative to self._basedir

Fixes #11789

test/units/mock/loader.py

@@ -29,11 +29,11 @@ class DictDataLoader(DataLoader):
     def __init__(self, file_mapping=dict()):
         assert type(file_mapping) == dict
 
+        super(DictDataLoader, self).__init__()
+
         self._file_mapping = file_mapping
         self._build_known_directories()
 
-        super(DictDataLoader, self).__init__()
-
     def load_from_file(self, path):
         if path in self._file_mapping:
             return self.load(self._file_mapping[path], path)