Use junit to provide a temporary directory for testing

So far the java.nio.Files was used to create a cache directory during
tests. Junit also provides a functionality to create temporary
directories for tests.

This change implements the usage of a junit rule to create a temporary
directory. This also increases the similarity to the HA-plugin.

Change-Id: Id184b12ab1abf6d0ca355c41f3313ff583c916a3