net: move testHookSetKeepAlive into hook.go

Change-Id: I1f2d4e3b0351a7a47c3a6073833a17dbc0c7b05c
Reviewed-on: https://go-review.googlesource.com/8520
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/net/hook.go b/src/net/hook.go
index 14dc19f..32ba15e 100644
--- a/src/net/hook.go
+++ b/src/net/hook.go
@@ -4,4 +4,7 @@
 
 package net
 
-var testHookLookupIP = func(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) { return fn(host) }
+var (
+	testHookLookupIP     = func(fn func(string) ([]IPAddr, error), host string) ([]IPAddr, error) { return fn(host) }
+	testHookSetKeepAlive = func() {}
+)