fullblocktests: Add additional tests
fullblocktests: Add additional tests

* Orphan with null prev hash
* Valid orphan due to undelivered block
* Duplicate regular block
* Duplicate orphan block

jrick/dcrd,jcmincke/dcrd,jcmincke/dcrd,jrick/btcd,decred/dcrd,jrick/dcrd,jrick/btcd,decred/dcrd,jrick/btcd,jcmincke/dcrd