Empty Extension file can be left out

[HermanPeeren]

You don't need to add the CmsMigratorComponent: if it is empty, you can leave it out. The system will then automagically instantiate an MVCComponent.

You gave it a BootableExtensionInterface, but the boot() method is empty. All unnecessary. You might think the BootableExtensionInterface is necessary to boot a component, but that is not true (in fact the name of the interface is confusing). It only means, that you have a boot() method in your extension in which you can put some extra stuff that will be done during booting the extension. In the Application object, bootComponent() is called, which calls $this->loadExtension(). In that method (on line 179-181 of the ExtensionManagerTrait) you can see:

    if ($extension instanceof BootableExtensionInterface) {
          $extension->boot($container);
      }

In short: you don't need the CmsMigratorComponent and it doesn't need to implement the BootableExtensionInterface.

Of course you must delete the CmsMigratorComponentTest too, but that was not useful anyway (for instance: you were checking if the BootableExtensionInterface was implemented, while that is not necessary at all).