[TASK] Add unit tests for stdWrap_substring()

Resolves: #71772
Releases: master
Change-Id: I8ca9ab566f3e429c93b64c44bb0697c0b24f96e3
Reviewed-on: https://review.typo3.org/44899


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>