Add visual tests for field components readonly + value

[web-padawan]

Describe your motivation

Follow-up from https://github.com/vaadin/web-components/pull/7204#discussion_r1523410953

Currently, we only have visual tests for readonly state without value set. It would be good to extend those.

Describe the solution you'd like

Provide the following combinations:

• readonly
• readonly with value
• readonly with invalid and value