Ext Reg Integration: document new `test` hook

An extension might have a custom test-runner, and custom test runners should not be added to the `test` npm script in package.json anymore since it will not be picked up by the new `aio app test` functionality.

For it to be run by `aio app test`, it has to be added to the `hooks` property of the extension's `ext.config.yaml` under a `test` key.

e.g.
```
hooks:
test: 'echo my custom test runner`
```

The docs here have to be updated to reflect the new app hooks location in the extension's `ext.config.yaml` as well:
https://www.adobe.io/apis/experienceplatform/project-firefly/docs.html#!AdobeDocs/project-firefly/master/guides/app-hooks.md