Issue #2482693 by prics, drunken monkey: Fixed and completed database backend tests.

search_api_db/src/Tests/BackendTest.php

src/Query/Query.php

src/Query/QueryInterface.php

src/Tests/ExampleContentTrait.php

...nfig/install/field.field.entity_test.article.category.yml

.../config/install/field.field.entity_test.item.category.yml

..._db/config/install/field.storage.entity_test.category.yml

...config/install/search_api.index.database_search_index.yml