net/http: deflake TestTransportPersistConnLeak on macOS

On a loaded system, sometimes connections don't work out.
Ignore those in TestTransportPersistConnLeak to avoid flakes.

For #33585.

Change-Id: Ic07057532dc0ea5115d6ec49c3c29099a9382295
Reviewed-on: https://go-review.googlesource.com/c/go/+/261538


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Russ Cox <rsc@golang.org>