Add correct typing to test examples

After ethereum/execution-spec-tests#2281, all python functions in this repository will require to have their return type defined, which means that all pytest test functions, which do not return anything, need to specify `-> None`.

Example:
```python
@pytest.mark.with_all_tx_types
@pytest.mark.valid_from("Berlin")
def test_something_with_all_tx_types(
state_test: StateTestFiller,
pre: Alloc,
tx_type: int
):
pass
```
should now be:
```python
@pytest.mark.with_all_tx_types
@pytest.mark.valid_from("Berlin")
def test_something_with_all_tx_types(
state_test: StateTestFiller,
pre: Alloc,
tx_type: int
) -> None:
pass
```

An easy way to identify these to grep for `def test` in files that are not `*.py`.