newlines in strings are invalid JSON

Although script_valid.json and script_invalid.json are loaded correctly by the JSON interpreter used by bitcoin core, these same files are often used by other libraries and do not necessarily load correctly due to the fact that newlines contained inside strings are not valid and must instead use the escape character \n. The files tx_valid.json and tx_invalid.json handle this correctly, so I've changed the formatting in script_valid.json and script_invalid.json to mirror those files.

If you want to see the error, try loading these files in node before and after this fix, e.g.:

```
require('script_valid');
```