test(mesh): add mesh coloring test

New 20-line test file for mesh coloring:
- Tests that create_coloring produces valid graph coloring
- Validates that elements in same color don't share nodes
- Uses Abaqus mesh file (cube_tet4.inp) for testing

Essential for parallel element assembly where elements sharing
nodes must be processed in different colors to avoid race conditions.

test/mesh/test_coloring.jl

@@ -0,0 +1,20 @@
+using JuliaFEM, LinearAlgebra, Test
+
+datadir = first(splitext(basename(@__FILE__)))
+
+@testset "Test that mesh coloring provides a good coloring"
+    fn = joinpath(datadir, "cube_tet4.inp")
+    mesh = JuliaFEM.Mesh(open(parse_abaqus, fn))
+
+    coloring = JuliaFEM.create_coloring(mesh)
+    for colors in coloring
+        for ele_i in colors
+            for ele_j in colors
+                if ele_i == ele_j
+                    continue
+                end
+                @test intersect(Set(mesh.elements[ele_i]), Set(mesh.elements[ele_j])) |> isempty
+            end
+        end
+    end
+end