Fixup plugin set test

tests/mozdef_util/test_plugin_set.py

@@ -12,9 +12,11 @@ class TestPluginSet(object):
         }
 
     def test_registered_plugins(self):
-        total_num_plugins = len([name for name in os.listdir(self.plugin_dir)])
-        # We exclude the __init__.py file
-        assert len(self.plugin_set.enabled_plugins) == total_num_plugins - 1
+        total_num_plugins = 0
+        for name in os.listdir(self.plugin_dir):
+            if name.startswith('plugin'):
+                total_num_plugins += 1
+        assert len(self.plugin_set.enabled_plugins) == total_num_plugins
 
     def test_registered_plugins_specific_enabled_plugins(self):
         enabled_plugins = ['plugin1']