mining: Add basic priority calculation tests.
mining: Add basic priority calculation tests.

This adds some basic tests for the priority calculation code in the
mining policy.

cqswxj2013/btcd,tb00/btcd,dajohi/btcd,stevenroose/btcd,ppcsuite/ppcd,davecgh/btcd,Roasbeef/btcd,ltcsuite/ltcd,wallclockbuilder/btcd,dajohi/btcd,ltcsuite/ltcd,Roasbeef/btcd,tb00/btcd,cqswxj2013/btcd,ppcsuite/ppcd,btcsuite/btcd,stevenroose/btcd,btcsuite/btcd,Stash-Crypto/btcd,aakselrod/btcd,adiabat/btcd,jongillham/btcd,wallclockbuilder/btcd,Stash-Crypto/btcd,davecgh/btcd,adiabat/btcd,jongillham/btcd,dajohi/btcd,aakselrod/btcd