test: use faster variant for rss test-memoryusage-emfile

PR-URL: #36768
Refs: #34291
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

test/parallel/test-memory-usage-emfile.js

@@ -14,5 +14,5 @@ const files = [];
 while (files.length < 256)
   files.push(fs.openSync(__filename, 'r'));
 
-const r = process.memoryUsage();
-assert.strictEqual(r.rss > 0, true);
+const r = process.memoryUsage.rss();
+assert.strictEqual(r > 0, true);