autotests: set fixed locale forobjecttreeparsertest, rendertest, unencryptedmessagetest, also some brush over
Some more tests rely on data for a given locale, so set it for them: objecttreeparsertest, rendertest, unencryptedmessagetest
Also for consistent code experience some cleanup done before:
- Use QTest's initMain() feature over undocumented Q_CONSTRUCTOR_FUNCTION (later is undocumented)
- Use consistently qputenv instead of setenv
- use consistently LC_ALL=en_US.UTF-8 (limited C/POSIX locale charset not ensured by our code)