net: TestSendfile does not use sendfile on Windows

[neild]

https://go.dev/cl/581778 extends the existing TestSendfile* tests in the net package to check to see if sendfile (or the platform equivalent) is actually used.

It appears that in the case of TestSendfile on Windows, we aren't using TransmitFile. Other tests, such as TestSendfileParts are using TransmitFile. I'm not sure what's going wrong. I'll make the test skip the check on Windows for now; filing a bug to keep track of the problem.