st/test-theme: Use the gsettings memory backend

Mutter test context now make sure that they are run with the gsettings
memory backend to avoid the local environment to affect the tests.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2628>

src/st/meson.build

@@ -204,7 +204,8 @@ if get_option('tests')
   )
 
   test('CSS styling support', test_theme,
-    workdir: meson.current_source_dir()
+    workdir: meson.current_source_dir(),
+    env: ['GSETTINGS_BACKEND=memory']
   )
 endif