Anonymous positional splats cannot be directly accessed, they can
only be passed as splats to other methods.  So if an anonymous
positional splat would be empty, you can use a shared frozen
empty array to save an allocation.

```ruby
def a(*) end
a()
```

This is similar to how anonymous empty keyword splats are optimized,
except those use `nil` instead of a shared empty frozen hash.

This updates the allocation tests to check that the array allocations
are avoided where possible.

It also makes a small change to test_iseq.rb to ensure an unfrozen
hash is passed as the value of an anonymous splat parameter.