Image cache randomly updates

[germa89]

On https://github.com/ansys/pymapdl/pull/3386 the image cache got updated, but "in theory" no plotting dependencies were updated.

Well... in that PR, the visualizer was updated. Later I remove that update.

So it seems we are not pinging properly the important dependencies for testing. Since pyvista started to be a "subdependency" (dependency of a dependency, visualizer), I have seen this twice.... I did fix it first time in: https://github.com/ansys/pymapdl/pull/3371/files

I believe I should add pyvista as a test dependency so we make sure we are getting the same stuff everytime.