[TASK] Add tests for pages.slug in workspaces tests

In order to see the current behaviour in
workspaces when modifying pages, the slug
field is added to the existing "Regular"
DataHandler tests within workspaces.

Resolves: #87904
Releases: master, 9.5
Change-Id: Ie2432fb078f2b04515a24de7b525b1c151186b05
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60242


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>