TestCase: set locale from the environment

TestCase: set locale from the environment

This fixes an annoyance I'd had for a while: UTF-8 arrows etc.
in debug messages came out as "?", because until we call setlocale()
we're in the C locale, whose character set is ASCII (and some layer
of library responds by transcoding our strings into ASCII).

With this change, running the tests in en_GB.UTF-8 results in
Unicode arrows and more comprehensible output.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=696179
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>