Use Path.touch to create files for rmtree tests

It's not necessary to use `write_bytes(b"")`, because pathlib.Path
has `touch()`.

test/test_util.py

@@ -46,7 +46,7 @@ def permission_error_tmpdir(tmp_path):
     """Fixture to test permissions errors in situations where they are not overcome."""
     td = tmp_path / "testdir"
     td.mkdir()
-    (td / "x").write_bytes(b"")
+    (td / "x").touch()
 
     # Set up PermissionError on Windows, where we can't delete read-only files.
     (td / "x").chmod(stat.S_IRUSR)
@@ -73,7 +73,7 @@ def test_deletes_nested_dir_with_files(self, tmp_path):
             td / "s" / "y",
             td / "s" / "z",
         ):
-            f.write_bytes(b"")
+            f.touch()
 
         try:
             rmtree(td)
@@ -95,7 +95,7 @@ def test_deletes_dir_with_readonly_files(self, tmp_path):
         for d in td, td / "sub":
             d.mkdir()
         for f in td / "x", td / "sub" / "y":
-            f.write_bytes(b"")
+            f.touch()
             f.chmod(0)
 
         try:
@@ -115,7 +115,7 @@ def test_avoids_changing_permissions_outside_tree(self, tmp_path):
 
         dir1 = tmp_path / "dir1"
         dir1.mkdir()
-        (dir1 / "file").write_bytes(b"")
+        (dir1 / "file").touch()
         (dir1 / "file").chmod(stat.S_IRUSR)
         old_mode = (dir1 / "file").stat().st_mode