test: update tests to be more scalable

[jackwotherspoon]

Current integration tests use a bad practice of initializing the Connector
within the getconn creator func. This creates a Connector on each
database conn instead of re-using a single connector:

def getconn() -> pg8000.dbapi.Connection:
    # initialize Connector object for connections to Cloud SQL
    with Connector() as connector:

This PR also makes a single test file be able to have tests connect to
different Cloud SQL instances (wanted for CAS tests) by removing
the pytest fixture in place of helper func.

[enocom]

Thanks for cleaning these up.