Improve testing for KPP to avoid issues like the one Alistair discovered in commit ee75da9ca7... two things came up in that test:

1) The bug occurred when the OBL depth was above the center of the level containing the boundary layer, but we were only testing a case where the OBL depth was below the level center
2) The bug occured in the viscosity output (now Mdiff output), and we were only outputtingdiffusivity (now Tdiff).

So Test 4 is split into Test 4a and 4b, where the former looks at OBL_depth=13.75 and the latter looks at OBL_depth=11.25, both outputting all 3 diffusivity arrays. The old test only considered OBL_depth=14; the change from 14 -> 13.75 crops up because I changed the namelist variable from OBL_depth to the index of the level containing the OBL_depth; the actual boundary layer depth is halfway between the cell center and the cell interface (bottom interface for 4a, top interface for 4b).

Note that the bug Alistair found was fixed in 8ae608d3a2 when I updated variable names.