os: skip Lchown test on Android if symlink doesn't work

After upgrading builder device (android/arm) to android 5.0.2,
the test started failing. Running 'ln -s' from shell fails with
permission error.

Change-Id: I5b9e312806d58532b41ea3560ff079dabbc6424e
Reviewed-on: https://go-review.googlesource.com/22962Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_unix_test.go

@@ -145,6 +145,9 @@ func TestLchown(t *testing.T) {
 
 	linkname := f.Name() + "2"
 	if err := Symlink(f.Name(), linkname); err != nil {
+		if runtime.GOOS == "android" && IsPermission(err) {
+			t.Skip("skipping test on Android; permission error creating symlink")
+		}
 		t.Fatalf("link %s -> %s: %v", f.Name(), linkname, err)
 	}
 	defer Remove(linkname)