Support running multiple instances of the test suite in parallel

Originally created by @intrigeri on #9486 (Redmine)

Make sure our infrastructure can cope with running more tests, and run multiple instances of the test suite in parallel, which will follow from automation.

This may imply:

• to set up more test VMs (as a follow-up on tails/sysadmin#6564);
• to adjust the Puppet module (already created via tails/sysadmin#5683) and manifests to support multiple parallel test runners;
• to design and implement a way to fire up a clean test runner VM before each test suite run.

Parent Task: tails/sysadmin#5288

Subtasks

• tails/sysadmin#9646
• tails/tails#10215

Related issues

• Related to tails/tails#10117
• Related to tails/sysadmin#10601
• Related to tails/sysadmin#11295
• Blocked by tails/sysadmin#9645