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