[skip ci][WIP] Add tests for all facdir/facemod variants

test/sumfact/poisson/CMakeLists.txt

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

...t/poisson/facedir-facemod-variation/generate_meshfiles.py

test/sumfact/poisson/facedir-facemod-variation/grid_00.msh

test/sumfact/poisson/facedir-facemod-variation/grid_01.msh

test/sumfact/poisson/facedir-facemod-variation/grid_02.msh

test/sumfact/poisson/facedir-facemod-variation/grid_03.msh

test/sumfact/poisson/facedir-facemod-variation/grid_04.msh

test/sumfact/poisson/facedir-facemod-variation/grid_05.msh

test/sumfact/poisson/facedir-facemod-variation/grid_06.msh

test/sumfact/poisson/facedir-facemod-variation/grid_07.msh

test/sumfact/poisson/facedir-facemod-variation/grid_08.msh

test/sumfact/poisson/facedir-facemod-variation/grid_09.msh

test/sumfact/poisson/facedir-facemod-variation/grid_10.msh

test/sumfact/poisson/facedir-facemod-variation/grid_11.msh

test/sumfact/poisson/facedir-facemod-variation/grid_12.msh

test/sumfact/poisson/facedir-facemod-variation/grid_13.msh

test/sumfact/poisson/facedir-facemod-variation/grid_14.msh

test/sumfact/poisson/facedir-facemod-variation/grid_15.msh

test/sumfact/poisson/facedir-facemod-variation/grid_16.msh