test(topology): add hexahedra test

Test file included in main test/runtests.jl
Tests hexahedral element topology

test/topology/test_hexahedra.jl

@@ -0,0 +1,38 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using JuliaFEM
+using Test
+using StaticArrays
+
+@testset "Hexahedron Elements" begin
+    @testset "Hexahedron{8}" begin
+        @test nnodes(Hexahedron{8}()) == 8
+        @test dim(Hexahedron{8}()) == 3
+        @test nvertices(Hexahedron{8}()) == 8
+        @test nedges(Hexahedron{8}()) == 12
+        @test nfaces(Hexahedron{8}()) == 6
+        
+        # Entity counts
+        @test nentities(Hexahedron{8}, Vertex) == 8
+        @test nentities(Hexahedron{8}, Edge) == 12
+        @test nentities(Hexahedron{8}, Face) == 6
+        @test nentities(Hexahedron{8}, Cell) == 1
+    end
+    
+    @testset "Hexahedron{20}" begin
+        @test nnodes(Hexahedron{20}()) == 20
+        @test nvertices(Hexahedron{20}()) == 8  # Same topology
+        @test nedges(Hexahedron{20}()) == 12
+        @test nfaces(Hexahedron{20}()) == 6
+        @test nentities(Hexahedron{20}, Edge) == 12
+    end
+    
+    @testset "Hexahedron{27}" begin
+        @test nnodes(Hexahedron{27}()) == 27
+        @test nvertices(Hexahedron{27}()) == 8  # Same topology
+        @test nedges(Hexahedron{27}()) == 12
+        @test nfaces(Hexahedron{27}()) == 6
+        @test nentities(Hexahedron{27}, Edge) == 12
+    end
+end