build: Include test-theme in tests

It's an existing test that doesn't require interaction, so there's
no reason why it shouldn't be run by `ninja test`.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/419

src/st/meson.build

@@ -139,9 +139,14 @@ test_theme = executable('test-theme',
   sources: 'test-theme.c',
   c_args: st_cflags,
   dependencies: [clutter_dep, gtk_dep],
+  build_rpath: mutter_typelibdir,
   link_with: libst
 )
 
+test('CSS styling support', test_theme,
+  workdir: meson.current_source_dir()
+)
+
 libst_gir = gnome.generate_gir(libst,
   sources: st_gir_sources,
   nsversion: '1.0',