Unit tests should be migrated to `scenario`

### Context

Current unit tests are using harness to simulate Juju environment. This library has been deprecated in favor of [scenario](https://documentation.ubuntu.com/ops/latest/reference/ops-testing/). Unit tests need to be migrated.

### What needs to get done

1. Migrate unit tests to `scenario`

### Definition of Done

1. Unit tests are passing

Locate the unit tests that currently use the deprecated harness, then read the linked scenario documentation to understand the replacement testing model. Migrate the affected tests to scenario and run the unit test suite; done means all unit tests pass.

Written by the indexing model from the issue text.