Try harder to fix focus tests

The focus test now asserts that its window is active.
So we should not run them in parallel with other tests,
since those might steal the focus.

testsuite/gtk/meson.build

@@ -261,6 +261,7 @@ foreach test : focus_chain_tests
       join_paths(meson.current_source_dir(), 'focus-chain', test[0] + '.ui'),
       join_paths(meson.current_source_dir(), 'focus-chain', test[0] + '.' + test[1]),
     ],
+    is_parallel: false,
     env: test_env,
     suite: [ 'gtk', 'focus' ],
   )