Try giving 6 vcpus to each isotester on lizard

Originally created by @intrigeri on #10971 (Redmine)

See https://gitlab.tails.boum.org/tails/tails/-/issues/9264#note_81240. The goals are:

1. see if this makes the test suite run faster, that is, if we could run the test suite more often on lizard v2;
2. see if this increases global CPU usage consumption on the host (we’re currently wasting quite a lot of CPU cores that are idling).

Parent Task: tails/tails#9264

Related issues

• Related to tails/tails#6729