Patterns and content only: add E2E test coverage for basic functionality

[ramonjd]

For the Pattern Editing and contentOnly interactivity feature, let's add some ground-level E2E coverage.

• Inserted patterns' content fields (and no others) are editable by default
• Entering and existing pattern editing mode

This is just to provide a foundation level for later.

[talldan]

These tests were added in Pattern editing: stabilize and remove the experiment flag, so I'll close the issue.