Safepoint deadlock(s)

[rmacnak-google]

A handful of multi-isolates tests are flaky timing out during app-jit training runs. The app-jit training might be having an effect because it implies --deterministic and no background compiler.

vm/dart/spawn_shutdown_test
vm/dart/isolates/ring_gc_test
vm/dart/isolates/fibonacci_call_test

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