test: add missing unit tests

[sebastiendan]

Description

This PR adds the missing frontend and backend unit tests. Additionally, it reworks the test CI workflow and adds a code coverage workflow.

Fixes TOO-342

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works

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