benchmark: always throw the same Error instance

Stack trace capturing currently accounts for 40 % of the benchmark
running time. Always throwing the same exception object removes
that overhead and lets the benchmark be more focused on what it is
supposed to measure.

Refs: nodejs#34512 (comment)

benchmark/async_hooks/promises.js

@@ -31,10 +31,11 @@ const bench = common.createBenchmark(main, {
   ]
 });
 
+const err = new Error('foobar');
 async function run(n) {
   for (let i = 0; i < n; i++) {
     await new Promise((resolve) => resolve())
-      .then(() => { throw new Error('foobar'); })
+      .then(() => { throw err; })
       .catch((e) => e);
   }
 }