tests: Use G_DEBUG=fatal-warnings here too

I am trying to track down a warning I see in `test-keyfile-utils`
which turned out to be the installed case only, but let's inject
this here too.

(The GLib default is broken, but it's hard to fix upstream without
 breaking the world)

Closes: #610
Approved by: jlebon

Makefile-tests.am

@@ -28,6 +28,7 @@ EXTRA_DIST += \
 # include the builddir in $PATH so we find our just-built ostree
 # binary.
 TESTS_ENVIRONMENT += OT_TESTS_DEBUG=1 \
+	G_DEBUG=fatal-warnings \
 	GI_TYPELIB_PATH=$$(cd $(top_builddir) && pwd)$${GI_TYPELIB_PATH:+:$$GI_TYPELIB_PATH} \
 	LD_LIBRARY_PATH=$$(cd $(top_builddir)/.libs && pwd)$${LD_LIBRARY_PATH:+:$${LD_LIBRARY_PATH}} \
 	PATH=$$(cd $(top_builddir)/tests && pwd):$${PATH} \