test: remove test-child-process-fork-args flaky designation

test-child-process-fork-args does not appear in results from `ncu-ci
walk commit` and no one has reported it failing for several months.
A stress test also passed.

Closes: https://github.com/nodejs/node/issues/32863

PR-URL: https://github.com/nodejs/node/pull/34684
Fixes: https://github.com/nodejs/node/issues/32863


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>