Fix path in test message (#4102)

Author: JelleZijlstra

tests/test_black.py

@@ -317,7 +317,7 @@ def test_expression_diff(self) -> None:
  msg = (
  "Expected diff isn't equal to the actual. If you made changes to"
  " expression.py and this is an anticipated difference, overwrite"
- f" tests/data/expression.diff with {dump}"
+ f" tests/data/cases/expression.diff with {dump}"
  )
  self.assertEqual(expected, actual, msg)