benchmark: fix EADDRNOTAVAIL error in http2

Fixes: https://github.com/nodejs/node/issues/34061

It is found that there may be errors caused by too many TCP connections. This PR is only to reduce the number of connections to avoid benchmark tests crashing. On my macOS, when the value of duration is 2, there will be no errors basically.

/cc @addaleax @BridgeAR @jasnell

Checklist

• tests and/or benchmarks are included
• commit message follows commit guidelines