removes extra plugin calls

When we gather the plugins, we go through two list comprehensions,
but call them each time. So on the second pass, the plugins that
were called the first time can't be called again. This change
makes it so that all fo the plugins are called together in the
second list comprehension.

lib/matplotlib/__init__.py

@@ -1550,7 +1550,7 @@ def test(verbosity=1, coverage=False):
 
         # store the old values before overriding
         plugins = _get_extra_test_plugins()
-        plugins.extend([plugin() for plugin in nose.plugins.builtin.plugins])
+        plugins.extend([plugin for plugin in nose.plugins.builtin.plugins])
 
         manager = PluginManager(plugins=[x() for x in plugins])
         config = nose.config.Config(verbosity=verbosity, plugins=manager)