dg : add a 2-triangles square mesh for testcases

Solver/TESTCASES/square2.msh

+$MeshFormat
+2.2 0 8
+$EndMeshFormat
+$PhysicalNames
+2
+1 1 "Border"
+2 2 "Inside"
+$EndPhysicalNames
+$Nodes
+4
+1 0 0 0
+2 1 0 0
+3 1 1 0
+4 0 1 0
+$EndNodes
+$Elements
+6
+1 1 2 1 1 4 3
+2 1 2 1 2 3 2
+3 1 2 1 3 2 1
+4 1 2 1 4 1 4
+5 2 2 2 6 1 2 3
+5 2 2 2 6 1 3 4
+$EndElements