The CompressibleTpfa class always passes a non-null `forces->wells'
object to the constructor, assembly, and reconstruction routines but
uses ``forces->wells->W == 0'' to signify a simulation model without
wells. This is, arguably, an error in the CompressibleTpfa class but
one that does not require a lot of work to support in the
cfs_tpfa_residual module.
Insert the extra tests in an effort to honour the ``liberal in what you
accept, strict in what you produce'' principle.