Migrate unit tests from `ops.harness`

[mvlassis]

Context

ops.testing.Harness is considered deprecated since https://documentation.ubuntu.com/ops/latest/reference/ops-testing-harness/. The replacement for it should be state-transition tests.

The ops team has created a migration guide which should be helpful

What needs to get done

• Update all units tests under tests/unit to move away from ops.Harness.

Definition of Done

• Our unit tests don't use ops.harness

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-8512.

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