Merge branch 'cleanup/tests' into 'master'

test/heatequation/heatequation_dg.mini

test/heatequation/heatequation_dg.ufl

test/laplace/laplace_dg.ufl

test/nonlinear/CMakeLists.txt

test/nonlinear/nonlinear.mini

test/nonlinear/nonlinear_dg.mini

test/nonlinear/nonlinear_dg.ufl

test/nonlinear/nonlinear_dg_matrix_free.mini

test/nonlinear/nonlinear_matrix_free.mini

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