The test_duplicate_with_pruned_ancestor test needs to get around a
limitation where the shreds with a parent older than the latest root are
discarded. The previous approach manually adjusted the root value in the
blockstore; this is not ideal in that it is fiddling with the inner
working of Blockstore.
So, use the is_trusted argument in Blockstore::insert_shreds(); setting
is_trusted=true bypasses the sanity checks (including the parent >=
latest root check).