Enable fiber dumps on Scala Native

[armanbilge]

Everything is already implemented and they are working correctly AFAIK.

I made an attempt to re-enable them in #3809 but we were hit by out-of-memory problems in CI.

I'm not sure if we are just stressing CI and/or Scala Native GC too much in our test suite ... or if there is still a legitimate bug with WeakReferences that prevents them from being collected in a timely manner.