Add custom plugins tests (#5383)

* add tests for the plugin API

* make plugin invocation optional

It could be that an object has been passed as a plugin, in that case we
will use the `handler` function from the object. If it doesn't exist,
then we will only take the `config` section out of it.

src/lib/setupContextUtils.js

@@ -499,7 +499,7 @@ function registerPlugins(plugins, context) {
         pluginItem(pluginApi)
       }
     } else {
-      plugin(pluginApi)
+      plugin?.(pluginApi)
     }
   }