[BUGFIX] Stabilize backend user acceptance test

With #103692 remembering the sub-module route within the backend user
module has been fixed to open the last opened module sub-route.

Due to another splitting of composer acceptance tests, the backend
user groups tests are executed before the backend user tests within
the same instance - and thus staying on the backend user groups view.

`ListUserCest` does not ensure to select the backend user lists like
it is done for `ListGroupCest`.

This change ensures to switch to the backend users sub-route, when the
`backend user` module is selected in the `ListUserCest::_before()`
similar to `ListGroupCest::_beore()`.

Resolves: #103837
Related: #103692
Releases: main, 12.4
Change-Id: If54812a7bfc95e317f5e72737c3f88461da3fda2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84278


Tested-by: core-ci <typo3@b13.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>