I haven't tried it myself, but my understanding is that Spirea are non-toxic. There are a bunch of variety of spirea. I looked up yours...Goldmound's scientific name is Spirea japonica. While I couldn't find that listed specifically, I founded many trusted sources that say Spirea is non-toxic. My guess is your tortoise is pretty safe. If you haven't seen any ill effects by now, you're probably not going to.
Note to anyone who is interested in knowing if a plant is edible - Put your plant's name in the Google search bar and ask if it's toxic. Choose the answer that comes up the most often. I tend to trust the sites that refer to dogs and cats, as the research is much more common for warm blooded animals than cold.