Approval checking unit tests (#3252)

* node/approval_checking: break out filled_tranch_iterator method

In the subsequent commit, we will begin to test this method in
isolation.

* node/approval-voting: fix tranche back-filling algorithm

Previously, this algorithm would generate duplicate, empty entries for
tranches (1..pre_end). This is caused because the initial value (0) for
gap_end is treated as the end of a prior tranche that wasn't actually
processed. The first pass thus would add (1..tranche) empty entries, in
addition to the (0..pre_end) empty entries chained at the end of the
method.

This is fixed by using the current tranche as the gap_start for the
first iteration, ensuring that the approval_entries_filled only produces
entries in the range (pre_end..post_start).

* Address feedback