doc: recommend node-stress-single-test for flaky tests

Add guidance to the collaborator guide and onboarding docs recommending
node-stress-single-test when fixing flaky tests, so contributors verify
stability under repeated runs.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/64223
Refs: https://openjs-foundation.slack.com/archives/C03BJP63CH0/p1782716150521499
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Stewart X Addison <sxa@redhat.com>
Reviewed-By: Richard Lau <richard.lau@ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

doc/contributing/collaborator-guide.md

@@ -296,7 +296,7 @@ label to the pull request.
 
 * [`node-stress-single-test`](https://ci.nodejs.org/job/node-stress-single-test/)
   can run a group of tests over and over on a specific platform. Use it to check
-  that the tests are reliable.
+  that the tests are reliable (i.e. not flaky).
 
 * [`node-test-commit-v8-linux`](https://ci.nodejs.org/job/node-test-commit-v8-linux/)
   runs the standard V8 tests. Run it when updating V8 in Node.js or floating new

onboarding.md

@@ -286,7 +286,10 @@ needs to be pointed out separately during the onboarding.
 * If you are interested in helping to fix coverity reports consider requesting
   access to the projects coverity project as outlined in [static-analysis][].
 * If you are interested in helping out with CI reliability, check out the
-  [reliability repository][] and [guide on how to deal with CI flakes][].
+  [reliability repository][] and [guide on how to deal with CI flakes][]. When
+  fixing a flaky test, it is recommended to run [`node-stress-single-test`][]
+  on both the main branch and the test branch to verify the fix makes the test
+  more stable under repeated runs.
 
 [Code of Conduct]: https://github.com/nodejs/admin/blob/HEAD/CODE_OF_CONDUCT.md
 [Labels]: doc/contributing/collaborator-guide.md#labels
@@ -296,6 +299,7 @@ needs to be pointed out separately during the onboarding.
 [`author-ready`]: doc/contributing/collaborator-guide.md#author-ready-pull-requests
 [`core-validate-commit`]: https://github.com/nodejs/core-validate-commit
 [`git-node`]: https://github.com/nodejs/node-core-utils/blob/HEAD/docs/git-node.md
+[`node-stress-single-test`]: https://ci.nodejs.org/job/node-stress-single-test/
 [`node-test-pull-request`]: https://ci.nodejs.org/job/node-test-pull-request/
 [guide on how to deal with CI flakes]: https://github.com/nodejs/test?tab=readme-ov-file#protocols-in-improving-ci-reliability
 [participants' expenses]: https://github.com/openjs-foundation/cross-project-council/blob/main/community-fund/COMMUNITY_FUND_POLICY.md#community-fund-rules