Flaky Byron ThreadNet failure: ImmutableDB is leaking file handles

[amesgen]

On current main (44d8867):

 $ cabal run byron-test -- --quickcheck-replay="(SMGen 3815441042141447428 1971304436672976329,3)" -p '/simple convergence/'
      Node CoreId (CoreNodeId 0)'s ImmutableDB is leaking file handles
      1 /= 0

This bug seems to have been introduced/revealed by #1228; the command above doesn't failure before it (i.e. it fails on af19eff, but not on d6e35dc).

Maybe #1228 is only related insofar as it lead to the exploration of a different STM schedule exhibiting a bug that was already present, just not found before. Also see #1105 (comment).