[libcxx][test] Portability fix for some locale-dependent tests.

Tests for `std::system_error` constructor marked as slightly non-portable.
One (but not the only one) reason for such non-portability is that these
tests assume the default locale to be set to "C" (or "POSIX").

However, the default locale for the process depends on OS and
environment. This patch adds explicit setting of the correct
locale expected by the tests.

Thanks to Andrey Maksimov for the patch.

Differential Revision: https://reviews.llvm.org/D72456