Move P2P's split_by_* tests

[hendrikmakait]

Minor refactoring.

• Tests added / passed
• Passes pre-commit run --all-files

[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 8m 12s ⏱️ + 1h 18m 10s
 4 051 tests  -     6   3 953 ✅ +   10     97 💤  -   9  1 ❌  - 3 
51 787 runs  +7 566  49 771 ✅ +7 340  2 015 💤 +246  1 ❌  - 3 

For more details on these failures, see this check.

Results for commit c763a21. ± Comparison against base commit 5a588ae.

This pull request removes 18 and adds 12 tests. Note that renamed tests count towards both.

distributed.protocol.tests.test_arrow
distributed.protocol.tests.test_collection
distributed.protocol.tests.test_highlevelgraph
distributed.protocol.tests.test_numpy
distributed.protocol.tests.test_pandas
distributed.shuffle.tests.test_graph
distributed.shuffle.tests.test_merge
distributed.shuffle.tests.test_merge_column_and_index
distributed.shuffle.tests.test_metrics
distributed.shuffle.tests.test_rechunk
…

distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler_report_args[False]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_scheduler_report_args[report_args0]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[1]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[False]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers[True]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers_report_args[False]
distributed.diagnostics.tests.test_memray ‑ test_basic_integration_workers_report_args[report_args0]
distributed.shuffle.tests.test_shuffle ‑ test_split_by_partition[False]
distributed.shuffle.tests.test_shuffle ‑ test_split_by_partition[True]
…