Flaky tests

When running PowerMock tests in our CI system, we discovered a few tests that would fail intermittently, attached diffs fix the flakiness (I see you found one of them too). 

Additionally, one of the tests had an implicit assumption that it runs under Surefire, so we had to get rid of that as well (we use buck). 

We don't run the full set of tests (just the junit (rule|runner) + mockito + (objenesis|xstream) combos), so there are likely other issues remaining.

Lastly, I don't know if TestNG makes any guarantees on the method execution order, so I didn't fix the static initializer testng version as well.