Currently Cockroach isn't performant for concurrent database setup and
tear-down. Instead of a single instance allow setting multiple potential
connection strings and let the tests pick one connection string
randomly.
This improves test duration by ~10 minutes.
While we are at significantly changing how pgtest works, introduce
helper PickPostgres and PickCockroach for selecting the database to
reduce code duplications in multiple places.
Change-Id: I8ad171d5c4c8a4fc081ec2ae9bdd0cc948a80619
