Originally posted by @flying-sheep in #20842 (comment)

Hi, I’m unsure what to do regarding testing. I guess “just add a unit test” is probably correct, but I didn’t find clear instructions. Maybe this could be improved?

1. CONTRIBUTING.md links to the wiki main page. It doesn’t have in-text links to anything about contributing or testing, but there’s a sidebar with all wiki pages. Maybe it should mention the right page to go to when trying to contribute?

2. The wiki page Submitting a pull request doesn’t mention what tests to add. Maybe it should say “Go to the Testing page and add one or more tests that best fit your change”?

3. The wiki page Testing mentions a lot of options in great detail, but not what exactly to do when creating a PR. Maybe it should do that?

4. The check output is also not very clear: This “Check for changed files” run says

   TypeScript code was edited without also editing a "src//*.test.ts\nsrc//*.testvirtualenvs.ts\n.github/test_plan.md" file; see the Testing page in our wiki on testing guidelines (the "skip tests" label can be used to pass this check)

   The file pattern in there implies I should ask a test plan, but from context I assume this means that I should add a unit test (or in case my change wouldn’t be testable, add a test plan document), is that right? Maybe the file pattern should be removed or explained