Add canDecode flag to tests.
Add canDecode flag to tests.

This commit adds a new flag to the tests which controls whether or not an
address can be decoded.  This is to support the upcoming public key
address type and possible future addresses which aren't directly
decodable.

conseweb/coinutil,jrick/dcrd,jrick/btcd,CryptocurrencyCabal/btcutil,jrick/dcrd,decred/dcrd,jrick/btcutil,FactomProject/btcutil,ppcsuite/ppcutil,jrick/btcutil,ppcsuite/ppcutil,wallclockbuilder/btcutil,btcsuite/btcutil,CryptocurrencyCabal/btcutil,jrick/btcd,FactomProject/btcutil,Roasbeef/btcutil,dajohi/btcutil,pkothbauer/btcutil,benma/btcutil,Roasbeef/btcutil,pkothbauer/btcutil,davecgh/btcutil,btcsuite/btcutil,jbenet/btcutil,jrick/btcd,wallclockbuilder/btcutil,benma/btcutil,decred/dcrd,dajohi/btcutil,jbenet/btcutil,davecgh/btcutil,conseweb/coinutil