doc: fix link in `test/README.md`

[LiviaMedeiros]

The relative link points to wrong place when the page is opened as https://github.com/nodejs/node/tree/main/test (without trailing slash nor explicit README.md), because new URL('./common/README.md', 'https://github.com/nodejs/node/tree/main/test').href === 'https://github.com/nodejs/node/tree/main/common/README.md'.

Note that there are similar links in other places (example: https://github.com/nodejs/node/blob/103b8439cae2a113f4f0796dd6fcbced294a3ed8/test/wpt/README.md?plain=1#L8), but they work fine because GitHub resolves them internally to absolute links. I guess it just doesn't touch links in footnotes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.24%. Comparing base (103b843) to head (8f1492c). Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #55165   +/-   ##
=======================================
  Coverage   88.23%   88.24%           
=======================================
  Files         651      651           
  Lines      183863   183863           
  Branches    35824    35827    +3     
=======================================
+ Hits       162235   162245   +10     
+ Misses      14932    14923    -9     
+ Partials     6696     6695    -1     

see 24 files with indirect coverage changes

[github-actions[bot]]

Fast-track has been requested by @RedYetiDev. Please 👍 to approve.

[avivkeller]

Fast-tracking, as this is an extremely simple change, and shouldn't need to much deliberation?

[nodejs-github-bot]

Landed in 0f02810fc940ca16dd4e3794f6e96dd5c0eb6ba0