Use platform dirs in tests (#872)

* use platform dirs in tests

* set env variable sooner

* add note

Author: blink1073

tests/conftest.py

@@ -3,6 +3,10 @@
 import sys
 
 import pytest
+
+# Must be set before importing from `jupyter_core`.
+os.environ['JUPYTER_PLATFORM_DIRS'] = '1'
+
 from jupyter_core import paths
 
 from .utils import test_env