Add missing mini file

test/nonlinear/nonlinear_dg_matrix_free.mini

+__name = nonlinear_dg_matrix_free_{__exec_suffix}
+__exec_suffix = numdiff, symdiff | expand num
+
+lowerleft = 0.0 0.0
+upperright = 1.0 1.0
+elements = 4 4
+elementType = simplical
+
+[wrapper.vtkcompare]
+name = {__name}
+extension = vtu
+
+[formcompiler]
+numerical_jacobian = 1, 0 | expand num
+matrix_free = 1
+exact_solution_expression = g
+compare_l2errorsquared = 5e-3
+
+# Disable numerical methods (not working in PDELab?)
+{__exec_suffix} == numdiff | exclude