test: document NODE_TEST_KNOWN_GLOBALS

PR-URL: #27434
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/common/README.md

@@ -9,6 +9,7 @@ This directory contains modules used to test the Node.js implementation.
 * [Countdown module](#countdown-module)
 * [DNS module](#dns-module)
 * [Duplex pair helper](#duplex-pair-helper)
+* [Environment variables](#environment-variables)
 * [Fixtures module](#fixtures-module)
 * [Heap dump checker module](#heap-dump-checker-module)
 * [HTTP2 module](#http2-module)
@@ -495,6 +496,17 @@ which returns an object `{ clientSide, serverSide }` where each side is a
 
 There is no difference between client or server side beyond their names.
 
+## Environment variables
+
+The behavior of the Node.js test suite can be altered using the following
+environment variables.
+
+### NODE_TEST_KNOWN_GLOBALS
+
+A comma-separated list of variables names that are appended to the global
+variable whitelist. Alternatively, if `NODE_TEST_KNOWN_GLOBALS` is set to `'0'`,
+global leak detection is disabled.
+
 ## Fixtures Module
 
 The `common/fixtures` module provides convenience methods for working with