Add a functional test covering #8127

[mpdude]

Before merging, let's wait what conclusions #10411 or #10389 make.

This PR adds a functional test covering the use case reported in #8127.

It uses joined table inheritance across a hierarchy of three entity classes, with the root and middle class even being abstract entity classes. All three classes contain a mapped field.

As long as all three entity classes are declared in the Discriminator Map, the leaf (concrete entity) class can be queried through either the root or leaf class name, and all fields will be initialized correctly.

When the middle class is not included in the DM, its fields will not be queried and the entity is not hydrated correctly.

When the middle class is not an @Entity class, but an abstract @MappedSuperclass, it need not be included in the DM for things to work.