internal/keyspan: add interleaving iterator test implementation and crosscheck

To better test against problems like https://github.com/cockroachdb/pebble/pull/5743, we should add a toy testing implementation that deals with small integer keys only (and perhaps use `[]bool` to mark tombstoned areas), and cross-check the real implementation using random ops.

We can apply upper and lower bounds by chopping up the list of keys/tombstones (we chop at the upper bound on SeekGE, we chop at the lower bound on SeekLT).

Jira issue: PEBBLE-1333