os: make TestMkdirAllWithSymlink more robust

Don't assume the test has a clean environment within /tmp.
Use an actual new tempdir for its tests.

Fixes FreeBSD build failure as seen at:
http://build.golang.org/log/396738676356d7fb6bab6eaf1b97cac820f8a90f

--- FAIL: TestMkdirAllWithSymlink (0.00 seconds)
path_test.go:178:                 Mkdir /tmp/dir: mkdir /tmp/dir: file exists
FAIL
FAIL    os      1.091s

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6615057

src/pkg/os/path_test.go

@@ -5,6 +5,7 @@
 package os_test
 
 import (
+	"io/ioutil"
 	. "os"
 	"path/filepath"
 	"runtime"
@@ -171,20 +172,23 @@ func TestMkdirAllWithSymlink(t *testing.T) {
 		return
 	}
 
-	tmpDir := TempDir()
+	tmpDir, err := ioutil.TempDir("", "TestMkdirAllWithSymlink-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tmpDir)
+
 	dir := tmpDir + "/dir"
-	err := Mkdir(dir, 0755)
+	err = Mkdir(dir, 0755)
 	if err != nil {
 		t.Fatalf("Mkdir %s: %s", dir, err)
 	}
-	defer RemoveAll(dir)
 
 	link := tmpDir + "/link"
 	err = Symlink("dir", link)
 	if err != nil {
 		t.Fatalf("Symlink %s: %s", link, err)
 	}
-	defer RemoveAll(link)
 
 	path := link + "/foo"
 	err = MkdirAll(path, 0755)