Elements init_expr with get_global

Right now the following is invalid in the spec interpreter with error `test.wast:4.21-4.23: invalid module: unknown global 0`
```
(module
  (table 1 anyfunc)
  (global $g i32 (i32.const 0))
  (elem (get_global $g))
)
```

It is valid using wabt wast2wasm.
It also made me realize that there are no tests in this repo testing elements with an init_expr using a global