core/txbuilder: make tests faster
core/txbuilder: make tests faster

Don't use pgtest.NewDB where it's not necessary.
Also don't use the database at all in protocol.Chain.
This speeds up 'go test chain/core/txbuilder' from
~5s to ~3s on my machine.

Closes chain/chainprv#1607.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jbowens




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain