Automatic test: don't include any deb.torproject.org experimental APT source

Let’s avoid issues like #15978 (closed) and #16883 (closed) in the future.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/37#note_108671

Related issues

• Related to #15978 (closed)
• Related to #16883 (closed)
• Blocks #16209

Original created by @intrigeri on 16931 (Redmine)