TestTemplatesModel, TestGenerationTest: C locale to avoid translated strings

Summary:
KConfig reads translated strings out of desktop files, using default
QLocale. Tests check against untranslated strings though.

Test Plan: tests no longer fail e.g. with a LANG=de_DE.UTF-8 env

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3980