[BUGFIX] Add correct diff source values to translated record

This change ensures that the original language record diff source
(usually in the field l10n_diffsource as configured by
transOrigDiffSourceField) is set correctly as soon as a localized
record is created.

To guard this, an additional test for pages is added, and several
DataHandler tests are adjusted.

TYPO3 displays the source language value of a field for a translated
record. This information field can display a diff, if the source record
value was changed. To accomplish this a shadow copy of localizable
fields from the source language record is saved in the configurable
language diff source field.

Localization of a record creates a new record, adding the values of
localizable fields value with a prefix and setting needed system fields
like parent record id, language id and so on. This is updated if a
localized field is changed in the localized language.

`\TYPO3\CMS\Core\DataHandling\DataHandler` as the main actor in the
backend is respons...