Fix flaky test_restart_waits_for_new_workers

[crusaderky]

https://github.com/dask/distributed/pull/8571/checks?check_run_id=22534355076

>       assert s.workers.keys().isdisjoint(original_workers.keys())
E       AssertionError: assert False

[github-actions]

Unit Test Results

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

    27 files  ±0      27 suites  ±0   9h 52m 28s ⏱️ - 30m 9s
 4 050 tests ±0   3 938 ✅ ±0    110 💤 ±0  2 ❌ ±0 
50 854 runs  ±0  48 518 ✅ ±0  2 334 💤 ±0  2 ❌ ±0 

For more details on these failures, see this check.

Results for commit 1dcd851. ± Comparison against base commit 8a1ecb8.

[milesgranger]

Nice, thanks @crusaderky!