Add 'set_dagster_home' flag to instance_for_test

Summary: Example of where you might want this if you have a test that sets up two different instances and you want one to be the 'true' one that is returned when you set DagsterInstance.get()

Test Plan: BK

Reviewers: alangenfeld, johann, max, sashank

Reviewed By: johann

Differential Revision: https://dagster.phacility.com/D8477