[GeoMechanicsApplication] Add new DGeoFlow C++ tests for the new piping element

[rfaasse]

The DGeoFlow C++ tests use the 'old' piping element. Before the DGeoFlow team switches to the new one, we should have tests in place for the C++ workflow.

Acceptance Criteria

• The C++ tests for DGeoFlow are converted (or copied and converted) to the 'new' piping element.

Update March 31, 2025 [avdg81]

The mdpa files with the new line elements have already been prepared previously. We can reuse them in this issue.