[FIX] hr_timesheet: allow leaves to be modified

- The HR staff may not be able to accept/refuse/cancel leaves if the
  module `project_timesheet_holidays` is installed.

  This is due to the commit c9da1571
  introducing a manual check on the current user's group without
  checking if the current environment is in superuser mode.

- Also fix the test verifying the access restriction, which meant to use
  a different user, but did that with sudo(), obviously defeating
  the fix. sudo is not used to switch user anymore, that `with_user`.

closes odoo/odoo#55274

X-original-commit: 75a166f1d115cc712bb39e7711d0fcb173732650
Related: odoo/enterprise#12149
Signed-off-by: Olivier Dony (odo) <odo@openerp.com>