Add io tests that load WriteVTK with JuliaFEM and cover stub misuse, Hex8 and
Quad4 VTU output with nodal and cell data, suffix handling, and bad topologies.
- Add test/io/runtests.jl wrapper around the VTK-focused cases
- Implement test_writevtk.jl with structured mesh grids and negative cases
Jukka Aho