Implement Process Tests based on BPM Assert 3.0

[rseedorff]

Is your feature request related to a problem?
There is a new version of the BPM Assert Library. We should integrate and use this library to reimplement our process modell tests again:

• https://blog.camunda.com/post/2019/03/camunda-bpm-assert-300-alpha1-released/
• Compile Camunda BPM Assert for Java 7&8 and enable AssertJ 2&3 respectively camunda/camunda-bpm-assert#109

[rseedorff]

Engine "Prozess-Tests" auf neues Framework migrieren und wieder aktivieren