[Dexter] Add support for writing !step values
Following from the previous patch, this patch adds support to Dexter for
generating expected values for !step nodes. This is relatively limited:
the kind of !step which this is most well-suited to this is !step exactly,
as the !step order of ignoring extra lines is redundant (all lines are added
as expected values), and !step never can't know what lines could have been
stepped on but weren't without some extra work (e.g. finding viable
breakpoint locations in the enclosing state node).
