os: re-enable hostname test on openbsd

Now that Sysctl() works, re-enable the hostname test.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5408042

src/pkg/os/os_test.go

@@ -942,11 +942,6 @@ func TestHostname(t *testing.T) {
 		return
 	}
 
-	// TODO(jsing): Fix nametomib() on OpenBSD
-	if syscall.OS == "openbsd" {
-		return
-	}
-
 	// Check internal Hostname() against the output of /bin/hostname.
 	// Allow that the internal Hostname returns a Fully Qualified Domain Name
 	// and the /bin/hostname only returns the first component