cmd/migratedb: fix tests on first setup
cmd/migratedb: fix tests on first setup

No longer OK to assume that 'api-test' database exists.

Closes chain/chainprv#1033.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg

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