Set locale and timezone for Grantlee tests

Summary:
Those tests work with dates so it's important to make sure they are executed
with the same locale and timezone everywhere, otherwise the results won't
match the expected HTML templates (they seem to be failing for that very
reason the CI).

Reviewers: gjditchfield, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D22965