warpx unit test with image check

[nicolemarsaglia]

We can render streamlines but the input dataset particles have too tiny a radius for the raytracer to pick up their presence. To get an image I manually scaled up the data.

In order to do the necessary scaling from the unit test we should move it from tests/vtkh to tests/ascent to easily apply other pipelines.

https://github.com/Alpine-DAV/ascent/blob/develop/src/tests/vtkh/t_vtk-h_warpx_streamline.cpp#L167