test: mark `test-os-process-priority` flaky

[kfarnung]

The test also fails in upstream node on my machine, mark it flaky on Windows rather than skipping it.

Refs: https://github.com/nodejs/node/issues/22799

Checklist

• [x] make -j4 test (UNIX), or vcbuild test (Windows) passes
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message follows commit guidelines