doc: add missing function to test common doc

Add documentation for `common.expectsError()` to `test/README.md`.

PR-URL: https://github.com/nodejs/node/pull/11382


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>