Completed SonataAdminExtensionTest for full test coverage

Code coverage of SonataAdminExtension after this PR will be: 100%. (SonataAdminBundle
 lines coverage: 24.00%)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT