Fix flaky test_same_subnet_unsubscription()

[figtracer]

Issue Addressed

#7573

thread 'subnet_service::tests::test::test_same_subnet_unsubscription' (1640572) panicked at beacon_node/network/src/subnet_service/tests/mod.rs:356:13:
assertion `left == right` failed
  left: [Unsubscribe(Attestation(SubnetId(3)))]
 right: []
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Why

the race condition seems due to the usage of is_subscribed() instead of is_subscribed_permanent(), which was not following the comment's desired behaviour since is_subscribed() considers both permanent and temporary subscriptions.

// If we are permanently subscribed to this subnet, we won't see a subscribe message

so, previously, we could be taking the if branch for a temporary subscription

        if subnet_service.is_subscribed(&Subnet::Attestation(subnet_id1)) {
            // If we are permanently subscribed to this subnet, we won't see a subscribe message
            let _ = get_events_until_num_slots(&mut subnet_service, None, 1).await;

discarding Subscribe and potentially Unsubscribe events; thus, when reaching

        // Get event for 1 more slot duration, we should get the unsubscribe event now.
        let unsubscribe_event = get_events_until_num_slots(&mut subnet_service, None, 1).await;

the expected Unsubscribe could have been discarded already, resulting in [].

Fix

refactored the test to use a more deterministic approach following the other tests and removed is_subscribed() since we take the same route by checking is_subscribed_permanent() first

Results

Run #2000
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.77s
warning: the following packages contain code that will be rejected by a future version of Rust: num-bigint-dig v0.8.4
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
     Running unittests src/lib.rs (target/debug/deps/network-ecf0aa6b04bd6e2d)

running 1 test
test subnet_service::tests::test::test_same_subnet_unsubscription ... ok

successes:

successes:
    subnet_service::tests::test::test_same_subnet_unsubscription

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 71 filtered out; finished in 6.11s