SWIM: Unlock ignored_test_swim_shouldNotifyClusterAboutUnreachableNode_andThenReachableAgain

[ktoso]

I was unable to get the following test nice and reproducable so we now have an integration test with suspending the entire system to cover it.

Would be nice to revive:

    // FIXME: Can't seem to implement a hardened test like this...
     func ignored_test_swim_shouldNotifyClusterAboutUnreachableNode_andThenReachableAgain() throws {

Introduced in https://github.com/apple/swift-distributed-actors/pull/403/files?file-filters%5B%5D=.swift#diff-d008a60d3a0938d1713ab1369bd05b86R218-R262

[ktoso]

Revisit this as we extract it into its own repo; This will make it also easier to test I hope