Cirrus CI: run at most 3 Rust tests at a time
Cirrus CI: run at most 3 Rust tests at a time

This works around the same problem as described in #770, but for tests:
Cargo auto-detects the number of CPUs and spawns the corresponding
number of worker threads, but since Docker is limited in virtual CPUs
and RAM, this ends up over-saturating the CPU and prevents us from
lowering the RAM cap.

newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat