testsuite: Pass GDK_DEBUG=default-settings to css tests

Otherwise, settings might creep in and change css defaults.

testsuite/css/meson.build

@@ -6,6 +6,7 @@ csstest_env.set('G_TEST_BUILDDIR', meson.current_build_dir())
 csstest_env.set('GIO_USE_VFS', 'local')
 csstest_env.set('GSETTINGS_BACKEND', 'memory')
 csstest_env.set('G_ENABLE_DIAGNOSTIC', '0')
+csstest_env.set('GDK_DEBUG', 'default-settings')
 
 subdir('parser')
 subdir('nodes')