Test coverage of finite-volume operators is poor

[johnomotani]

The 'finite-volume' operators, FV::Div_a_Laplace_perp, FV::Div_par_K_Grad_par, FV::D4DY4, FV::D4DY4_Index, FV::Div_par, FV::Div_f_v, are not well tested. FV::Div_par_K_Grad_par and FV::Div_par are used only in test-snb, and the rest appear not to be tested at all. This may lead to bugs being missed, e.g. #1813 (comment) was only caught because it affected the operators used by test-snb.