webdav: disable TestDir on Plan 9

Updates golang/go#11453.

Change-Id: Ia3560d382daffde995e9824b924d1938f08e6e41
Reviewed-on: https://go-review.googlesource.com/12880
Reviewed-by: Nigel Tao <nigeltao@golang.org>

webdav/file_test.go

@@ -12,6 +12,7 @@ import (
 	"path"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -506,6 +507,10 @@ func testFS(t *testing.T, fs FileSystem) {
 }
 
 func TestDir(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("see golang.org/issue/11453")
+	}
+
 	td, err := ioutil.TempDir("", "webdav-test")
 	if err != nil {
 		t.Fatal(err)