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:

node/test/wpt/README.md

Line 8 in 103b843

See the json files in [the `status` folder](./status) for prerequisites,

), but they work fine because GitHub resolves them internally to absolute links. I guess it just doesn't touch links in footnotes.