test: Add integration tests scaffolding

[gruyaume]

Description

Add integration tests scaffolding:

• CI workflow that runs tests on every PR and push to main
• Minimal integration test that simply builds the charm, deploys it and waits for it to be active

Note

• More integration tests will come up as we improve the charm but at least we got the scaffolding here now.

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