string: Make tst-strerror/tst-strsignal unsupported if msgfmt is not installed

Without msgfmt libc.mo files are not generated and its loading failure
is silent ignored with xsetlocale.

Also unset LANGUAGE environment variable to avoid it taking precedence
when loading the message catalog.  Although not strictly required
(since the test is issued with test-container and it sets a strict
environment variable) it follows other tests that deal with
translation.

Checked on x86_64-linux-gnu.