add tests for invalid meshes and adjust tolerance a bit to comply with some extreme cases

Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Isotropic_remeshing/smoothing_impl.h

@@ -527,7 +527,7 @@ private:
 
 
         // check degenerate cases
-        double tolerance = 1e-5; // to think about it
+        double tolerance = 1e-3; // to think about it
 
         if ( edge1.squared_length()          < tolerance ||
              edge2.squared_length()          < tolerance ||

Polygon_mesh_processing/test/Polygon_mesh_processing/test_compatible_remeshing.cpp

@@ -49,7 +49,9 @@ int main(int argc, char* argv[]){
         "data/tetra3",
         "data/tetra4",
         "data/two_tris_collinear",
-        "data/U"
+        "data/U",
+        "data/elephant-invalid",
+        "data/mech-holes-shark-invalid"
     };