Fix flaky test_raise_on_incompatible_partitions

[crusaderky]

e.g. https://github.com/dask/distributed/pull/8569/checks?check_run_id=22516322786

E           AssertionError: Regex pattern did not match.
E            Regex: 'shuffling \\w* failed'
E            Input: 'shuffle_barrier failed during shuffle c593d0fd37e591da8ff25429ed7c9655'

[crusaderky]

This line is the important bit; the rest is just code hygene

[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   10h 17m 1s ⏱️ - 5m 36s
 4 050 tests ±0   3 938 ✅ ±0    110 💤 ±0  2 ❌ ±0 
50 854 runs  ±0  48 519 ✅ +1  2 333 💤  - 1  2 ❌ ±0 

For more details on these failures, see this check.

Results for commit d1a6b82. ± Comparison against base commit 8a1ecb8.

♻️ This comment has been updated with latest results.

[milesgranger]

LGTM, thanks @crusaderky!