No, I removed the related folder under out/Debug, recompile the test apk, timer.html is not in the test apk.
Now I use the following codes:
      'variables': {
        'apk_name': 'XWalkRuntimeClientEmbeddedTest',
        'java_in_dir': 'test/android/runtime_client_embedded/javatests',
        'is_test_apk': 1,
        'asset_location': '<(java_in_dir)/assets',
        'additional_input_paths': [
          '<(asset_location)/timer.html',
        ],
       },
timer.html is in the test apk.