Add helpers for testing Actions

[ioncreature]

For actions:

const action = await broker.getActionWithMocks('actionName', {singletons, actions, plugins);
expect(action(params)).toBeSomething();

For singletons:

const singleton = await broker.getSingletonWithMocks('singletonName', {singletons});
expect(singleton).toBeSomething();

For plugins:

const plugin = await broker.getPluginWithMocks('pluginName', {singletons})
expect(plugin(params)).toBeSomething();

[ioncreature]

Then implement tests in https://github.com/ioncreature/example-with-agata