test(io): exercise WriteVTK VTU export for structured meshes

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

test/io/runtests.jl

@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+using Test
+
+@testset "I/O (VTK export)" begin
+    include("test_writevtk.jl")
+end

test/io/test_writevtk.jl

@@ -0,0 +1,58 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+using Test
+using JuliaFEM
+using WriteVTK
+using Tensors: Vec
+
+@testset "write_vtu_mesh stub for non-mesh second argument" begin
+    @test_throws ErrorException write_vtu_mesh("x", 1.0)
+end
+
+@testset "write_vtu_mesh Hex8 + point/cell data" begin
+    mesh = create_structured_box_mesh(Hex8; xmin = 0.0, xmax = 1.0, nx = 2, ymin = 0.0, ymax = 1.0, ny = 1, zmin = 0.0, zmax = 1.0, nz = 1)
+    d = mktempdir()
+    base = joinpath(d, "hex8_out")
+    n = nnodes_total(mesh)
+    ne = nelements(mesh)
+    u = randn(3, n)
+    p = collect(1.0:n)
+    path = write_vtu_mesh(
+        base,
+        mesh;
+        point_data = (; displacement = u, pressure = p),
+        cell_data = (; elem_id = collect(1.0:ne)),
+    )
+    @test endswith(path, ".vtu")
+    @test isfile(path)
+end
+
+@testset "write_vtu_mesh strips .vtu suffix" begin
+    mesh = create_structured_box_mesh(Hex8; xmin = 0.0, xmax = 1.0, nx = 1, ymin = 0.0, ymax = 1.0, ny = 1, zmin = 0.0, zmax = 1.0, nz = 1)
+    d = mktempdir()
+    path = write_vtu_mesh(joinpath(d, "with_suffix.vtu"), mesh)
+    @test isfile(path)
+    @test occursin("with_suffix.vtu", path)
+end
+
+@testset "write_vtu_mesh Quad4" begin
+    mesh = create_structured_box_mesh(Quad4; xmin = 0.0, xmax = 1.0, nx = 2, ymin = 0.0, ymax = 1.0, ny = 2)
+    d = mktempdir()
+    path = write_vtu_mesh(joinpath(d, "quad"), mesh; point_data = (; s = zeros(Float64, nnodes_total(mesh))))
+    @test isfile(path)
+end
+
+@testset "write_vtu_mesh unsupported topology" begin
+    nodes10 = Vec{3,Float64}[
+        Vec(0.0, 0.0, 0.0), Vec(1.0, 0.0, 0.0), Vec(0.0, 1.0, 0.0), Vec(0.0, 0.0, 1.0),
+        Vec(0.5, 0.0, 0.0), Vec(0.5, 0.5, 0.0), Vec(0.0, 0.5, 0.0),
+        Vec(0.0, 0.0, 0.5), Vec(0.5, 0.0, 0.5), Vec(0.0, 0.5, 0.5),
+    ]
+    conn = NTuple{10,UInt32}[
+        ntuple(i -> UInt32(i), 10),
+    ]
+    mesh = Mesh{10,Tet10}(nodes10, conn)
+    d = mktempdir()
+    @test_throws ArgumentError write_vtu_mesh(joinpath(d, "t10"), mesh)
+end