tests: Fix GDateTime format tests on non-English locales
It seems that the test expects g_date_time_format to return formatted results in English, and there is no setlocale (LC_ALL, "") call in the file so the test does run in the default C locale. However, gettext seems to read the value of LC_MESSAGES from the environment by itself. Even if the value of LC_MESSAGES locale is C because of not calling setlocale, gettext still translates the name of the month according to the LC_MESSAGES environment variable, causing g_date_time_format_locale to fail on the "%b" test case because it cannot convert UTF-8 text returned by get_month_name_with_day to ASCII. To avoid the test failure, we set the LC_MESSAGES environment variable to C before format tests and restore it at the end of the function. https://bugzilla.gnome.org/show_bug.cgi?id=795165