Test: Improve scheduler testing suite

[badtst]

Tests are flacky, here are a few instances:

• https://github.com/pasqal-io/warden/actions/runs/32365641865/job/96414438145?pr=94
• https://github.com/pasqal-io/warden/actions/runs/32950449621/job/98120420096
• https://github.com/pasqal-io/warden/actions/runs/32956586699/job/98139469951
• https://github.com/pasqal-io/warden/actions/runs/32972424214/job/98189054743?pr=97#step:7:470
• https://github.com/pasqal-io/warden/actions/runs/33772105454/job/100704667948
• https://github.com/pasqal-io/warden/actions/runs/33896274071/job/101099539815
• https://github.com/pasqal-io/warden/actions/runs/34842292027/job/103969820137