Playwright tests for the ui_patterns_source field

Problem/Motivation

The ui_patterns_source field has kernel tests, but nothing covers its real life cycle in the browser: the widget AJAX form, the default value, the form display widget and a component reading the field.

Proposed resolution

We could add a field.spec.ts suite with four independent tests, all tagged @base:
- Life cycle: default value, add content, edit through the widget AJAX form, render, edit again.
- Widget settings.
- The ui_patterns_ui form display widget.
- A component reading the field through the "Value from the component in field" source, with cache propagation checked as anonymous.

The test field is an unlimited field_test_source with 2 items in ui_patterns_test_content, hidden in the view display.