Visualization unit tests fail when run individually

[jdavcs]

Verified on Python 3.5, 3.6, 3.7, 3.8, Linux and Mac, dev and 20.05 branches.

The error is raised inside the routes package, but is caused by the interdependence of Galaxy's unit tests.

test/unit/visualizations/plugins/test_VisualizationPlugin.py:92: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
lib/galaxy/visualization/plugins/plugin.py:159: in _build_render_vars
    visualization_plugin=self.to_dict(),
lib/galaxy/visualization/plugins/plugin.py:122: in to_dict
    'href'          : self._get_url()
lib/galaxy/visualization/plugins/plugin.py:128: in _get_url
    return url_for('visualization_plugin', visualization_name=self.name)
.venv/lib/python3.5/site-packages/routes/util.py:201: in url_for
    encoding = config.mapper.encoding
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <routes._RequestConfig object at 0x7f87e9c6a550>, name = 'mapper'

    def __getattr__(self, name):
>       return getattr(self.__shared_state, name)
E       AttributeError: '_thread._local' object has no attribute 'mapper'

.venv/lib/python3.5/site-packages/routes/__init__.py:15: AttributeError

[bgruening]

Fixed in #10162