Cleanup poisson tests

test/poisson/CMakeLists.txt

...son/dimension-grid-variations/poisson_1d_cg_interval.mini

...son/dimension-grid-variations/poisson_1d_dg_interval.mini

...sson/dimension-grid-variations/poisson_1d_dg_interval.ufl

...imension-grid-variations/poisson_2d_cg_quadrilateral.mini

...son/dimension-grid-variations/poisson_2d_cg_triangle.mini

...imension-grid-variations/poisson_2d_dg_quadrilateral.mini

...dimension-grid-variations/poisson_2d_dg_quadrilateral.ufl

...son/dimension-grid-variations/poisson_2d_dg_triangle.mini

...sson/dimension-grid-variations/poisson_2d_dg_triangle.ufl

...n/dimension-grid-variations/poisson_3d_cg_hexahedron.mini

...n/dimension-grid-variations/poisson_3d_dg_hexahedron.mini

...on/dimension-grid-variations/poisson_3d_dg_hexahedron.ufl

.../dimension-grid-variations/poisson_3d_dg_tetrahedron.mini

...n/dimension-grid-variations/poisson_3d_dg_tetrahedron.ufl

test/poisson/opcount_poisson_dg.ufl

test/poisson/poisson.mini

test/poisson/poisson_cellwise_constant.mini

test/poisson/poisson_dg.mini

test/poisson/poisson_dg.ufl