[TPU Provisioner] Delete node pool when JobSet is completed, failed, or deleted

[danielvegamyhre]

This PR includes the following changes:

• Update node reconciliation based deletion controller to delete node pool when JobSet is completed, failed, or deleted
• Refactor deletion controller integration tests to be more extendable so we can add new test cases more easily
• Move controller integration tests to a test/integration/controller directory. When we add the Job webhook, we can add integration tests in test/integration/webhooks. E2E tests can go in test/e2e. This will help keep things organized.

[danielvegamyhre]

/gcbrun

[danielvegamyhre]

/gcbrun