[BUGFIX] Parent localization pointer missing in IRRE tests

The parent localization pointer (in this very case
l18n_parent, instead of l10n_parent) is missing in
the DataSet scenarios and assertions.

Resolves: #56161
Releases: 6.2
Change-Id: Ieac6dd15d84f15488e0ee4690186efe2fb8dead6
Reviewed-on: https://review.typo3.org/27768
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader