Add checkbox-inputs fixture for DOM testing

[SherlockShemol]

Hey! 👋

I noticed there's no dedicated test fixture for checkbox inputs, while other input types like range-inputs, text-inputs, number-inputs all have their own.

This PR adds a checkbox-inputs fixture with 3 test cases:

• Controlled checkbox
• Uncontrolled checkbox
• Disabled checkbox

Nothing fancy, just filling a small gap in test coverage.