Add interface test for adding a duplicate driver.
Add interface test for adding a duplicate driver.

This commit adds an interface test to ensure that attempting to add a
duplicate driver for a given database type does not overwrite an existing
one.

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