OPENDJ-2460: fix pluggable backend unit tests

This was evil. TestNG has a bug which prevents method dependencies from
working with inheritance. When looking to see if another test method has
already completed it sometimes looks in the wrong sub-class instance,
when it should look in the current sub-class instance

I tried to refactor the JE/PDB tests to use @Factory for creating test
instances (one for PDB one for JE), but both instances end up being
invoked in parallel. In order to preserve the ordering I tried using the
"priority" @Test attribute, but it did not work.

I reordered the test methods and ran them sequentially, but TestNG
ignores the declaration order and just runs them in alphabetical order.
Finally, I refactored the tests so that they have no side-effects and
can be run in any order, which was probably the right thing to do
anyway.