Tweak event extender (tests)

- Inject contract, not implementation
- Do not dispatch event in test, let the core do that
- Ensure the relevant database tables are reset prior to the test
- Use correct parameter order for assertions

Refs #2097.

framework/core/src/Extend/Event.php

@@ -11,7 +11,7 @@ namespace Flarum\Extend;
 
 use Flarum\Extension\Extension;
 use Illuminate\Contracts\Container\Container;
-use Illuminate\Events\Dispatcher;
+use Illuminate\Contracts\Events\Dispatcher;
 
 class Event implements ExtenderInterface
 {