net: move cgo-using darwin test to separate package
It is a goal / requirement that nothing in the standard library
on darwin require cgo, and this test-only file makes net use
cgo on darwin. Move it elsewhere.
Change-Id: I6c11a8391d3913f73ce0098ba63b29adf5864f93
Reviewed-on: https://go-review.googlesource.com/c/go/+/549255 Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com> Reviewed-by: Bryan Mills <bcmills@google.com>
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