HHH-15065: Add minimal reproducer and a suggested fix for 5.6

[bwaldvogel]

The first commit introduces a minimal reproducer for HHH-15065. The test fails most of the times but with some luck it is green.

The second commit contains my suggestion how queries with entity graphs, having multiple attributes can be made fully deterministic.

See https://github.com/hibernate/hibernate-orm/pull/4767 for the corresponding test for the main branch.