chore(ProcessEngineRule): replace deprecated TestWatchman super class

TestWatchman is deprecated since JUnit 4.9. ProcessEngineRule therefore
did not support RuleChain. This commit fixes the problem by using the
new type TestWatcher as a super class.

Signed-off-by: Franziska Sauerwein <franziska.sauerwein@gmail.com>
Signed-off-by: Fabian Knittel <fabian.knittel@lettink.de>