out-of-source storage for test files

[nicolaslg]

Commit 96cc234 adds some vtk mesh files in test_sample that are quite big.
I recommend looking for another way to store our test files, else we will end up with a bloated repo.

[benzaouille]

I will remove them from gmds and place them in an other repo, thank you

[franck-ledoux]

Just took a look how to do that. I will try to provide it in our organization with different repo. It implies to make our CI evolve.