Fix test that passes on main anyway

Black searches parent directories if the common base of the given
sources doesn't contain a "project root marker" so simply setting
--stdin-filename to `../whatever.py` won't cause any problems. Instead
let's set --stdin-filename to `../src/a.py` as `src` has its own
pyproject.toml file. Currently on main, the detected root will be the
parent directory of `src` but with this PR it'll be `src` as expected.

tests/test_black.py

@@ -1398,8 +1398,8 @@ def test_find_project_root(self) -> None:
 
             with change_directory(test_dir):
                 self.assertEqual(
-                    black.find_project_root(("-",), stdin_filename="../whatever.py"),
-                    (root.resolve(), "pyproject.toml"),
+                    black.find_project_root(("-",), stdin_filename="../src/a.py"),
+                    (src_dir.resolve(), "pyproject.toml"),
                 )
 
     @patch(