Upgrade our isotesters to Stretch

Originally created by @intrigeri on #11739 (Redmine)

I expect we’ll do that as soon as Debian Stretch is released, or during its freeze.

Parent Task: tails/sysadmin#12160

Related issues

• Related to tails/sysadmin#10068
• Related to tails/sysadmin#13138
• Related to #17080 (closed)