test: use faster variant for rss in test-vm-memleak.js

PR-URL: #36769
Refs: #34291
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-vm-memleak.js

@@ -39,7 +39,7 @@ const interval = setInterval(function() {
   } catch {
   }
 
-  const rss = process.memoryUsage().rss;
+  const rss = process.memoryUsage.rss();
   assert.ok(rss < 64 * 1024 * 1024,
             `memory usage: ${rss} (${Math.round(rss / (1024 * 1024))} MB)`);