This is a copy of the existing test_l10n, modified to use LC_TIME
instead of LC_MESSAGES. It's not safe as each call to g_setenv() or
setlocale() could cause the test to crash; there is no safe way to
change a threaded process's environment, and a threaded process's locale
can only be safely changed using uselocale(), not with setlocale().
The calls to g_setenv() are definitely not needed on Linux. I wonder
whether removing these will break the test on other platforms?
The calls to setlocale() should be replaced by a dance of
uselocale() -> duplocale() -> newlocale() -> uselocale() on Linux. But
this is not portable and this is a cross-platform test. We would have to
make the test platform-specific to do this. macOS and at least FreeBSD
provide these functions via xlocale.h, but this isn't portable.
