Added API tests

> Requires https://github.com/ezsystems/ezpublish-kernel/pull/790
> Requires https://github.com/ezsystems/ezpublish-kernel/pull/789

Implements API tests.

Calls methods from a real query builder instance. Uses the `__toString()` export of queries to assert that the built query is valid.

Depends on a fork of ezpublish-api that has __toString() implemented for queries.

### TODO
- [ ] Remove dependency on fork when PR merged