[GeoMechanicsApplication] Move the integration tests for the DGeoFlow C++ route to python (same as is done for settlement)

[rfaasse]

As a developer, I would like the GTest executable to only contain actual unit tests, so that they run quickly and coverage gives an accurate overview of unit-tested functionality.

Therefore, I'd like to move the DGeoFlow C++ route tests to be started via python, the same as is now done for the settlement test, see #12865.

[avdg81]

Now that the settlement test has a way of running trough python, the 4 DGeoFlow tests should follow a similar pattern.