Flaky subnet subscription tests

[jimmygchen]

We've been seeing flaky tests in subnet services for quite some time, seems like there are some race there, causing flakiness, We should fix this as it's happening quite frequently.

    thread 'subnet_service::tests::test::test_same_subnet_unsubscription' panicked at beacon_node/network/src/subnet_service/tests/mod.rs:358:13:
    assertion `left == right` failed
      left: [Unsubscribe(Attestation(SubnetId(3)))]
     right: []
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

https://github.com/sigp/lighthouse/actions/runs/15477117552/job/43575102120

[DioBr4nd0]

hey @jimmygchen , Can you assign this to me?