#132 speed up tests by avoiding unneccessary compilations, allow parallel executions

Avoiding unneccessary compilations made the tests run in half the time on my machine.

Parallel executions can be started with ```mvn test ``` and:
*  ```-DforkCount``` > 1
* or: ```-Dparallel=classes``` and ```-DthreadCount``` > 1.

However... :wink: I did not see any speed improvements when running in parallel (quad-core, SSD). 

Using the MemFileManager was not that easily possible, as it's package-private. 

Any suggestions?