state_transition tool doesn't properly parse JSON tests

The tool `cmd/state-transition/state_transition` doesn't properly parse input files with JSON tests. It breaks even on the associated sample tests.

```shell
cmd/state-transition/state_transition cmd/state-transition/state_transition_sample1.json
transactionCollidingWithNonEmptyAccount_init:
Exception: [json.exception.type_error.305] cannot use operator[] with a string argument with array
```