Remove direct phpunit dependency

[swissspidy]

Description

Fixes #237

How has this been tested?

Screenshots If applicable, add screenshots to show the visual change.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[swissspidy]

Fixed in #270