net, os, internal/poll: test for use of sendfile

net, os, internal/poll: test for use of sendfile

The net package's sendfile tests exercise various paths where
we expect sendfile to be used, but don't verify that sendfile
was in fact used.

Add a hook to internal/poll.SendFile to let us verify that
sendfile was called when expected. Update os package tests
(which use their own hook mechanism) to use this hook as well.

For #66988

Change-Id: I7afb130dcfe0063d60c6ea0f8560cf8665ad5a81
Reviewed-on: https://go-review.googlesource.com/c/go/+/581778
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>