Increase test coverage

[beutlich]

[beutlich]

The general idea is good, but:

* Would like some feedback on the enum.

I am not sure if ModelicaFileType is meant to be public. For testability I simply copied it.

* In case an assert triggers the program will not clean up afterwards. Fixing that would be complicated, as the used primitives (e.g., ModelicaInternal_mkdir) can also fail. Just a comment saying that in case the test fails you should delete directories abc and xyz before attempting again might suffice.

Indeed, the current testing setup is not good enough and does not support initialization or cleanup steps. A proper testing framework (e.g. Modelica_DeviveDrivers and ModelicaTableAdditions use GoogleTest) might solve the issue. But this is out of scope of this PR.