Fix/schema test isolation

[AlexanderMann]

Motivation

Presently, target-redshift has two minor bugs:

• doesn't use the redshift_schema information passed into it
• doesn't isolate instances of tests running against the remote Redshift cluster

The first is simple to fix. The second is damning because it prevents us from being able to have multiple CI or local tests running at the same time.

Suggested Musical Pairing

https://soundcloud.com/future-islands/ran