selftests/mount_setattr: add smoke tests for open_tree_attr(2) bug

There appear to be no other open_tree_attr(2) tests at the moment, but
as a minimal solution just add some additional checks in the existing
MOUNT_ATTR_IDMAP tests to make sure that open_tree_attr(2) cannot be
used to bypass the tested restrictions that apply to mount_setattr(2).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250808-open_tree_attr-bugfix-idmap-v1-2-0ec7bc05646c@cyphar.com


Signed-off-by: Christian Brauner <brauner@kernel.org>