doc: update table of contents for common/README.md

The `countdown` and `fixtures` modules are documented in the
`common/README.md` file but are omitted from the table of contents. Add
them.

PR-URL: https://github.com/nodejs/node/pull/15595
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common/README.md

@@ -6,6 +6,8 @@ This directory contains modules used to test the Node.js implementation.
 
 * [Benchmark module](#benchmark-module)
 * [Common module API](#common-module-api)
+* [Countdown module](#countdown-module)
+* [Fixtures module](#fixtures-module)
 * [WPT module](#wpt-module)
 
 ## Benchmark Module