At the moment, we only test inheritance via org.hibernate.ogm.datastore.mongodb.test.inheritance.SingleTableInheritanceTest

What we need to do is:

1. 1st commit: Move the classes under `org.hibernate.ogm.datastore.mongodb.test.inheritance` to a new package `org.hibernate.ogm.datastore.mongodb.test.inheritance.singletable`
2. 2nd commit Basically copy test and entities related to SingleTableInheritanceTest and create a new test class TablePerClassInheritanceTest under the package `org.hibernate.ogm.datastore.mongodb.test.inheritance.tableperclass`