test: document NODE_SKIP_FLAG_CHECK

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

@@ -501,6 +501,10 @@ There is no difference between client or server side beyond their names.
 The behavior of the Node.js test suite can be altered using the following
 environment variables.
 
+### NODE_SKIP_FLAG_CHECK
+
+If set, command line arguments passed to individual tests are not validated.
+
 ### NODE_TEST_KNOWN_GLOBALS
 
 A comma-separated list of variables names that are appended to the global