tests: Explicitly set TZ=UTC for g_time_val_from_iso8601() testing
tests: Explicitly set TZ=UTC for g_time_val_from_iso8601() testing

Some of the test vectors don’t specify a timezone, so the local one is
used; the comparisons to the expected results (which are in UTC) then
fail.

See discussion in
https://gitlab.gnome.org/GNOME/glib/merge_requests/225.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib