nettest: skip testConcurrentMethods on Plan 9

This test is running out of memory on Plan 9.

Updates golang/go#20489.

Change-Id: I79cdea9c31f396b6ef5c47474674ad736362b232
Reviewed-on: https://go-review.googlesource.com/44130Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

nettest/conntest.go

@@ -12,6 +12,7 @@ import (
 	"io/ioutil"
 	"math/rand"
 	"net"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -341,6 +342,9 @@ func testCloseTimeout(t *testing.T, c1, c2 net.Conn) {
 // testConcurrentMethods tests that the methods of net.Conn can safely
 // be called concurrently.
 func testConcurrentMethods(t *testing.T, c1, c2 net.Conn) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; see https://golang.org/issue/20489")
+	}
 	go chunkedCopy(c2, c2)
 
 	// The results of the calls may be nonsensical, but this should