ssh: specify InsecureIgnoreHostKey in the benchmarks

Change-Id: I06813be2b44ef8e5eb38fdac2d08a0f5cd840193
Reviewed-on: https://go-review.googlesource.com/86935
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/benchmark_test.go

@@ -40,7 +40,8 @@ func sshPipe() (Conn, *server, error) {
 	}
 
 	clientConf := ClientConfig{
-		User: "user",
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
 	}
 	serverConf := ServerConfig{
 		NoClientAuth: true,