scripts: update_chroot: start moving the tool from src/scripts/

Add a python entry point so we can migrate the src/scripts/ version.

BUG=b:298249751
TEST=CQ passes

Change-Id: I46827e8aec3fb6985048a6cc4c3e40a086089d5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4831960
Reviewed-by: Cindy Lin <xcl@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/update_chroot_unittest.py b/scripts/update_chroot_unittest.py
new file mode 100644
index 0000000..965901b
--- /dev/null
+++ b/scripts/update_chroot_unittest.py
@@ -0,0 +1,13 @@
+# Copyright 2023 The ChromiumOS Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Tests for update_chroot."""
+
+from chromite.scripts import update_chroot
+
+
+def test_main(run_mock):  # pylint: disable=unused-argument
+    """Smoke test."""
+    result = update_chroot.main([])
+    assert result == 0