Test coverage for each ScoreVerifier

As part of PLANNER-458, we've added ScoreVerifier implementations in optaplanner-test (for one-pager explanation, see new docs, last section in chapter 5).

Only HardSoftScoreVerifier and BendableScoreVerifier are currently indirectly unit tested through CloudBalancingScoreConstraintTest and TaskAssigningScoreConstraintTest.
The others are untested.

2 suggestions:

• We add unit test for each ScoreVerifier somehow
• or we add a ScoreConstrainTest for example (which we probably want to do anyway to test the examples DRL decently because they are used in demo's and copy-pasted by customers and users).