doc: update testing documentation

[Aditi-1400]

This update improves the test/common/README.md file by explaining why the common module is essential when writing tests.

I became curious about its importance while working on PR #56942, and @joyeecheung kindly provided the additional context that was missing from the documentation. :)

[joyeecheung]

LGTM, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.10%. Comparing base (048a17a) to head (2effd24).
Report is 69 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #56953 +/- ## ========================================== - Coverage 89.15% 89.10% -0.06%  ========================================== Files 665 665 Lines 192846 193206 +360 Branches 37145 37217 +72 ========================================== + Hits 171926 172150 +224 - Misses 13696 13760 +64 - Partials 7224 7296 +72 

see 51 files with indirect coverage changes

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/56953 ✔ Done loading data for nodejs/node/pull/56953 ----------------------------------- PR info ------------------------------------ Title doc: update testing documentation (#56953) ⚠ Could not retrieve the email or name of the PR author's from user's GitHub profile! Branch Aditi-1400:test-doc-update -> nodejs:main Labels test, needs-ci Commits 1 - doc: update testing documentation Committers 1 - Aditi <[email protected]> PR-URL: https://github.com/nodejs/node/pull/56953 Reviewed-By: Joyee Cheung <[email protected]> ------------------------------ Generated metadata ------------------------------ PR-URL: https://github.com/nodejs/node/pull/56953 Reviewed-By: Joyee Cheung <[email protected]> -------------------------------------------------------------------------------- ℹ This PR was created on Fri, 07 Feb 2025 20:42:22 GMT ✔ Approvals: 1 ✔ - Joyee Cheung (@joyeecheung) (TSC): https://github.com/nodejs/node/pull/56953#pullrequestreview-2602840643 ✘ This PR needs to wait 29 more hours to land (or 0 hours if there is one more approval) ✔ Last GitHub CI successful ✘ No Jenkins CI runs detected -------------------------------------------------------------------------------- ✔ Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/13311410448

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/56953 ✔ Done loading data for nodejs/node/pull/56953 ----------------------------------- PR info ------------------------------------ Title doc: update testing documentation (#56953) ⚠ Could not retrieve the email or name of the PR author's from user's GitHub profile! Branch Aditi-1400:test-doc-update -> nodejs:main Labels test, needs-ci Commits 1 - doc: update testing documentation Committers 1 - Aditi <[email protected]> PR-URL: https://github.com/nodejs/node/pull/56953 Reviewed-By: Joyee Cheung <[email protected]> Reviewed-By: Richard Lau <[email protected]> ------------------------------ Generated metadata ------------------------------ PR-URL: https://github.com/nodejs/node/pull/56953 Reviewed-By: Joyee Cheung <[email protected]> Reviewed-By: Richard Lau <[email protected]> -------------------------------------------------------------------------------- ℹ This PR was created on Fri, 07 Feb 2025 20:42:22 GMT ✔ Approvals: 2 ✔ - Joyee Cheung (@joyeecheung) (TSC): https://github.com/nodejs/node/pull/56953#pullrequestreview-2602840643 ✔ - Richard Lau (@richardlau) (TSC): https://github.com/nodejs/node/pull/56953#pullrequestreview-2615467845 ✔ Last GitHub CI successful ✘ No Jenkins CI runs detected -------------------------------------------------------------------------------- ✔ Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/13311572887

[aduh95]

Landed in 794f35f