Greeter revamp: Do user testing for "Welcome" and "Advanced"

Interesting testing points:

• “Start Tails” button, cf #8776 (closed).

Parent Task: #8230 (closed)

Related issues

• Related to #8776 (closed)
• Has duplicate #8234 (closed)
• Blocked by #7550 (closed)
• Blocks #8239 (closed)

Original created by @sajolida on 8238 (Redmine)