Fix `TestUptimeSignatures` flake

[alarso16]

Failing run

Whoever investigates this should also ensure it doesn't occur in coreth

[JonathanOppenheimer]

Here is another flake: https://github.com/ava-labs/subnet-evm/actions/runs/19945341593/job/57193282562?pr=1900

[JonathanOppenheimer]

Here is another flake: https://github.com/ava-labs/subnet-evm/actions/runs/19946351934/job/57196565110?pr=1900

[JonathanOppenheimer]

Here is another flake: https://github.com/ava-labs/subnet-evm/actions/runs/20043660623/job/57483859541?pr=1913