feat(fw,tests): define opcode exit statuses in fw libraries and use them consistently in tests

The EOF tests added these values to their spec (#834):

```python
LEGACY_CALL_FAILURE = 0
LEGACY_CALL_SUCCESS = 1
EXTCALL_SUCCESS = 0
EXTCALL_REVERT = 1
EXTCALL_FAILURE = 2
```

which makes the code more readable. It'd be nice to define these somewhere more central and use them consistently across all tests.