api/txdb/txdb_test: use separate schema from txdb
api/txdb/txdb_test: use separate schema from txdb

This prevents potential race conditions that can come about from
multiple packages sharing the same test schema. In each test package
init, the database schema is dropped and recreated. These events can
conflict if the schemas are not separate.

Closes chain/chainprv#651.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@jbowens

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