Add unit test for formatGettextDate()
Summary:
- gettextheader: Factorize date formatting into formatGettextDate()
- Add unit test for formatGettextDate()
Test Plan: make && ctest
Reviewers: aacid, sdepiets
Reviewed By: sdepiets
Subscribers: Localization
Differential Revision: https://phabricator.kde.org/D21842