test: fix multiple expectedWarnings bug

Commit 8fb4ea9f75 ("test: add deprecation code to expectWarning") did
not take into account that the same warning could be expected multiple
times. This bug was discovered in
https://github.com/nodejs/node/pull/18138 and this commit adds a fix for
this issue.

PR-URL: https://github.com/nodejs/node/pull/19766
Refs: https://github.com/nodejs/node/pull/18138
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common/index.js

@@ -631,7 +631,7 @@ function expectWarning(name, expected) {
     // Remove a warning message after it is seen so that we guarantee that we
     // get each message only once.
     map.delete(expected);
-  }, map.size);
+  }, expected.length);
 }
 
 function expectWarningByName(name, expected, code) {