update Test.ThreadNet.Util.Expectations for ChainSync's `k-1` edge case

The `Test.ThreadNet.Util.Expectations` hasn't changed since we noticed that ChainSync might not be able to recover from a `k`-deep fork, due to the ledger's forecast limit possibly not permitting it to fetch the `k+1`st header, which is necessary to trigger chain selection.

This Issue is to update that module to handle that case, or to prevent it in some other way, etc

This is an action item from Issue input-output-hk/ouroboros-consensus#641.