Add functional tests for admin deletion

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes

I also wanted to confirm that its correct that calling find(null, '/deleted/path') should throw a PHPCR exception. We found this in writing our own tests and it seems like it is not totally consistent with the way the ObjectManager works in the ORM and we expected it would just return null.