chore(dev): tune phpunit config to be more verbose and strict

[mfn]

Summary

At least as strict was we can currently tune it without breaking current tests.

This is light of #1152 to better see/show things like deprecation and other stuff we were not aware of.

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Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist:

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[mfn]

🤞🏼