[release-branch.go1.15] net: don't rely on system hosts in TestCVE202133195

Also don't unnecessarily deref the error return.

Updates #46504
Fixes #46531

Change-Id: I22d14ac76776f8988fa0774bdcb5fcd801ce0185
Reviewed-on: https://go-review.googlesource.com/c/go/+/324190


Trust: David Chase <drchase@google.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
(cherry picked from commit dd7ba3ba)
Reviewed-on: https://go-review.googlesource.com/c/go/+/324333


Reviewed-by: Ian Lance Taylor <iant@golang.org>