net: add TestSendfileParts

Add test for freebsd issue #25809.

This test also fails on my Windows 10 Version 1803.
My hope is that adding new test will break one of our builders.

Updates #25722
Updates #25809

Change-Id: Ia103bc708b8fa3b9af57613acc44893f90b3fa18
Reviewed-on: https://go-review.googlesource.com/117775


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>