Add STM tests that verify nested transactions are closed

STM implements "Closed Nested Transactions" as opposed to Flattened or Open. We need a test which demonstrates that nested transactions follow the closed model.

Furthermore, STM implementations can differ in how transactional code and non-transactional code are isolated from each other (with respect to visibility of updates during a transaction). Add a test that verifies that we use the "Weak Isolation" model.

Details of the expected behaviour is covered in the Sept 2018 jbossts blog entitled "Tips on how to evaluate STM implementations"