[TASK] Cover BackendUserAuthentication->returnWebmounts() with test

Also fix misleading comment about permissions.

Resolves: #91454
Releases: 9.5, master
Change-Id: I1a399f1be613f007440bf542441bee60f53e49e0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64557


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Richard Haeser <richard@maxserv.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Richard Haeser <richard@maxserv.com>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>