tests: replace rlimit dependency with rustix::process

[wtcpython]

rustix is already in our dependency tree; using it for test limits drops the standalone rlimit crate.

[Copilot]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.