sync/atomic: remove test dependency on net/http
sync/atomic: remove test dependency on net/http

Depending on net/http means depending on cgo.
When the tree is in a shaky state it's nice to see sync/atomic
pass even if cgo or net causes broken binaries.

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/10753044

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