processor: add tests.

@duncanbeevers , I wonder what do you think about stack processor rewrite and tests. Do you know about more stack examples that are worth to be tested?

I am not sure that this is what we want... Ideally file should be just `file:///C:/example.html`, but then we lose ` line 7 > eval line 1 > eval` context... Probably it should be:

```
        {
          "function": "at line 7 > eval line 1 > eval",
          "file": "file:///C:/example.html",
          "line": 1,
          "column": 1
        },
```

Thoughts?