This also removes the powermock dependency. Although powermock is a
fine library, it currently prevents dropping Junit4. And since we only
use the Whitebox API of powermock, this simply replaced powermock's
Whitebox with our own.
Although it it not that unreliable, it causes false negatives once in
a while. This is because the standard Java SE API does not provide a
way to force a *full* garbage collection run, we need to resort to
unreliable hacks to trigger one.
The test itself is still useful to diagnose or refute alleged memory
leaks.
This commit also move the test from JUnit 4 to Junit 5.
Florian Schmaus