Refactor some tests

[fjetter]

This refactors two tests that are very flaky.

test_deadlock_cancelled_after_inflight_before_gather_from_worker

(I was initially able to reproduce this when running the test a couple thousand times. The windows CI is really slow it seems)

and

test_restarting_during_transfer_raises_killed_worker

Both appear to be stable now on my machine

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    25 files  +    1      25 suites  +1   10h 25m 49s ⏱️ + 29m 15s
 4 121 tests  -     2   4 007 ✅  -     2    110 💤 ± 0  4 ❌ ±0 
47 598 runs  +2 655  45 506 ✅ +2 592  2 087 💤 +63  5 ❌ ±0 

For more details on these failures, see this check.

Results for commit 349136d. ± Comparison against base commit 928d770.

This pull request removes 2 tests.

distributed.tests.test_cancelled_state ‑ test_deadlock_cancelled_after_inflight_before_gather_from_worker[False-cancelled]
distributed.tests.test_cancelled_state ‑ test_deadlock_cancelled_after_inflight_before_gather_from_worker[False-resumed]