Mark 'Manager.all()` test as expected failure

Author: jacebrowning

tests/test_orm_usage.py

@@ -2,6 +2,8 @@
 
 from dataclasses import dataclass
 
+import pytest
+
 from datafiles import datafile
 from datafiles.utils import logbreak
 
@@ -69,6 +71,7 @@ class Bar:
     expect(bar.nested.value) == 2
 
 
+@pytest.mark.xfail(reason='https://github.com/jacebrowning/datafiles/issues/139')
 def test_values_are_filled_from_disk(expect):
     InventoryItem.objects.get_or_create(42, "Things", 0.99)