Add interface test for FetchBlockShaByHeight.
Add interface test for FetchBlockShaByHeight.

This commit adds an interface test for FetchBlockShaByHeight to ensure it
returns the same expected hash for each block inserted block height.

sysbot/btcd,Dirbaio/btcd,davecgh/btcd,toddfries/btcd,adiabat/btcd,Stash-Crypto/btcd,Dirbaio/btcd,jcmincke/dcrd,1d4Nf6/btcd,hectorj/btcd,Stash-Crypto/btcd,dajohi/btcd,Roasbeef/btcd,cjepson/btcd,jrick/btcd,danoctavian/btcd,decred/dcrd,btcsuitereleases/btcd,jrick/dcrd,soapboxsys/ombfullnode,hectorj/btcd,jcmincke/dcrd,davecgh/btcd,qchain/btcd,tb00/btcd,ppcsuite/ppcd-old,CryptocurrencyCabal/htcd,Cleawing/btcd,Dirbaio/btcd,ppcsuite/ppcd,decred/dcrd,ltcsuite/ltcd,DanielKrawisz/btcd,dajohi/btcd,cqswxj2013/btcd,jongillham/btcd,runeaune/btcd,toddfries/btcd,jimmysong/btcd,stevenroose/btcd,mathiasrw/btcd,runeaune/btcd,pkothbauer/btcd,danoctavian/btcd,btcsuite/btcd,ppcsuite/ppcd-old,1d4Nf6/btcd,jrick/btcd,cqswxj2013/btcd,sysbot/btcd,dan-da/btcd,BitGo/prova,qchain/btcd,btcsuite/btcd,pkothbauer/btcd,aakselrod/btcd,Cleawing/btcd,glv2/ppcd,adiabat/btcd,tb00/btcd,wallclockbuilder/btcd,cjepson/btcd,Roasbeef/btcd,dan-da/btcd,DanielKrawisz/btcd,btcsuitereleases/btcd,CryptocurrencyCabal/htcd,glv2/ppcd,mathiasrw/btcd,jimmysong/btcd,BitGo/prova,jongillham/btcd,jrick/dcrd,ppcsuite/ppcd,stevenroose/btcd,wallclockbuilder/btcd,jrick/btcd,FactomProject/btcd,ltcsuite/ltcd,jcmincke/dcrd,dajohi/btcd,aakselrod/btcd,soapboxsys/ombfullnode,FactomProject/btcd