math/rand uses a fixed seed, meaning that across runs the Kth call to
`rand.Int63()` will always return the same value.
Given that we want to provide a unique suffix across multiple
concurrent runs, this isn't the behavior we want.
I saw an instance fail in CI where all three legs ran the test
concurrently and they raced on creating the test stack, since they all
generated the same name.
8a6ffd08b2