core/txdb: make tests faster
core/txdb: make tests faster

Don't use pgtest.NewDB where it's not necessary.
This speeds up 'go test chain/core/txdb' from ~4s to ~2s
on my machine.

Closes chain/chainprv#1606.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jbowens




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