[skip ci] Use testtools to reduce code duplication in tests

.../sumfact/poisson/facedir-facemod-variation/CMakeLists.txt

...acedir-facemod-variation/poisson_dg_only_skeletons_3d.ufl

...tion/sumfact_poisson_dg_3d_facedir_facemod_variation.mini

...sumfact_poisson_dg_3d_facedir_facemod_variation_driver.cc

...umfact_poisson_dg_3d_facedir_facemod_variation_msh00.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh01.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh02.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh04.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh05.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh06.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh07.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh08.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh09.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh10.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh11.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh12.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh13.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh14.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh15.mini

...umfact_poisson_dg_3d_facedir_facemod_variation_msh16.mini