tests: properly set locale to C

We're testing for particular error messages, so we need to set to a C
locale to make sure we get the untranslated version.

Previously, this test set the LANG environment variable, but that's not
good enough if LANGUAGE is also set.  The only way to ensure that
LANGUAGE is ignored is to disable l10n with LC_ALL=C.
tests: remove assertion for '!uncertain' on .txt

Virtaal installs a mime package for various .po-like file formats, one
of which has the extension .txt.  This causes GLib to report ".txt"
files still as "text/plain" but no longer with complete certainty.

The result is that asserting !uncertain during the testsuite causes the
test to fail if Virtaal happens to be installed.

Remove this assertion.