You people should better stop testing your spoilers inside here. It could easily be construed as bumping, which is against the rules.
If you really want to test BB code functionality, you can do so even without actually posting anything.
Just click the "More Reply Options" button next to the button "Post" and you'll be presented not only additional means to spice up your posts but also a "Post Preview" right on top where you can see all BB code tags in action right away.
And for the records, you missed the closure part in your closing tag, like so "/spoiler".