test: use scenario for unit testing

[gruyaume]

Description

Here we replace harness with scenario for unit testing. Doing so, I identified an issue with self-signed certificates where the CA certificate expiry was not handled properly in charm code.

Rationale

The scenario framework enforces our current approach with:

• Arrange (state in)
• Act (juju event)
• Assert (state out)

This makes tests easier to read, to write, and to parameterise .

Reference

• https://github.com/canonical/ops-scenario

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library