Closes #880 Update tests base

[remyperona]

Description

Fixes #880

Type of change

• Chore

Detailed scenario

No scenario necessary, this is only changing testing code.

Technical description

Documentation

Update the tests base to use the v3 of our PHPUnit library, to bring it up to par with our other plugins.

New dependencies

wp-media/phpunit: 3

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.