rpcserver: Fix race in TestHandleTSpendVotes.
rpcserver: Fix race in TestHandleTSpendVotes.

This fixes a race that occurs when running TestHandleTSpendVotes test
cases in parallel.  It fixes the race by specifying a new *dcrutil.Block
per test since *dcrutil.Block can't be safely shared across goroutines.

decred/dcrd,decred/dcrd