GEODE-10021: Clean up RemoteTransactionDUnitTest (apache#7339)

Clean up RemoteTransactionDUnitTest for the next time this failure
occurs.

While trying to reproduce the failure described by GEODE-9929 I did some
clean up so I would be able to get more useful information out of the
failures. I cleaned up a lot of warnings, mostly using IntelliJ's tools.

I cleaned up:
- assertNull
- assertTrue
- assertFalse
- fail
- WaitCriterion
- added <Object> where required to get rid of warnings
- removed unnecessary throws of Exceptions on methods that don't throw
- in assertions switched the actual and expected to be the correct direction
- getGemfireCache (deprecated) -> getCache