Fix some tests to be compatible with `warn_missing_spec`

If `warn_missing_spec` is set then `make test` will fail with errors like:
```elixir
  3) test used_with_local_with_reattached_overridable (Kernel.WarningTest)
     test/elixir/kernel/warning_test.exs:299
     Assertion with == failed
     code: capture_err(fn -> Code.eval_string("defmodule Sample do\n  def hello, do: world\n  defp world, do: :ok\n  defoverridable [hello: 0, world: 0]\nend\n") end) == ""
     lhs:  "nofile:2: warning: missing specification for function hello/0\n"
     rhs:  ""
     stacktrace:
       test/elixir/kernel/warning_test.exs:300

```