tst-gettext2: make setup more robust

This test should be more robust about setting up its lang dirs.
I had two completely different systems (ia64 & x86_64) get wedged
in a way where the test just kept FAILing on me due to some of the
files missing.  This probably wasn't a big deal until the recent
commit which made checking of the locale dirs more robust (for
security reasons).
Regenerate libc.po