HHH-15854 Performance tests demonstrating regressions in Hibernate 6 (Branch 5.6)

[theigl]

This PR against the 5.6 branch serves to establish a performance baseline for the regressions described in HHH-15854.

https://github.com/hibernate/hibernate-orm/pull/6749 contains the same tests against the main branch.

https://hibernate.atlassian.net/browse/HHH-15854

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