Test new Hibernate ORM 6.3 API for batching

[DavideD]

We already test batching in some places:

• https://github.com/hibernate/hibernate-reactive/blob/main/hibernate-reactive-core/src/test/java/org/hibernate/reactive/BatchFetchTest.java
• https://github.com/hibernate/hibernate-reactive/blob/main/hibernate-reactive-core/src/test/java/org/hibernate/reactive/SubselectFetchTest.java

These are the new methods: #1747