Missing tests for `@SoftDelete`

[DavideD]

We have created a test for the support of @SoftDelete: SoftDeleteTest

The problem is that we only test entities that are mapped on a single table in the database. And, we don't test the use of @SoftDelete on associations.

We need additional tests with entities mapped using the different inheritance types.

As a guideline for first timers, to solve this issue one has to:

• [ ] Rename SoftDeleteTest to SoftDeleteSingleTableTest
• [ ] Mimic the existing test, but change the entities so that they use @Inheritance(strategy = InheritanceType.JOINED)
• [ ] Mimic the existing test, but change the entities so that they use @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
• [ ] Test @SoftDelete applied to a collection (might be a separate test class or a test method added to the previous one)
• [ ] Maybe it would helpful to also add a test for the other converters (now we only test the YesNoConverter)

I've collected everything in a single issue, but we could split this task in several issues when required.