[Core] test_reference_count.py, test_actor_constructor_borrow_cancellation, is really flaky

[rkooo567]

What happened + What you expected to happen

This is a real bug. The test checks if the actor who borrows a reference dies, if the borrowed reference is properly cleaned up. But the test is extremely flaky (almost half of time flaky).

We should fix it

Versions / Dependencies

master

Reproduction script

n/a

Issue Severity

None