Added ShowMapperTest, BaseMapperTest, BaseGroupedMapperTest

Possible BC break: Added some methods to AdminInterface.
Reason: ShowMapper (and some other parts of SonataAdmin) is using them, so I think they belongs there. Creating valid unit tests without this change will vbe very complicated.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a