tests: Set G_DEBUG environment variable

With us switching to glib more and more it is easy to get things
wrong (as can be seen in the previous commit). Set G_DEBUG
variable to "fatal-warnings" which causes GLib to abort the
program at the first call to g_warning() or g_critical().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

tests/meson.build

@@ -36,6 +36,7 @@ tests_env = [
   'abs_top_srcdir=@0@'.format(meson.source_root()),
   'LC_ALL=C',
   'LIBVIRT_AUTOSTART=0',
+  'G_DEBUG=fatal-warnings',
 ]
 
 if use_expensive_tests