Preact: Updating to use fixtures

Small shuffle of the code order so that Identify comes first. Also updated the tests to use fixtures :)

@amillet89 

@cgooley - I've left these fixtures pretty basic. You mentioned that you might be updating the integration to match your most recent API - if you do, just add your new fields to the fixtures as well as the mapper and you'll be good to go! You can also check out our [khaos templates](https://github.com/segmentio/integrations/blob/master/Contributing.md) to see the direction we're going in with standardising our code across integrations :) Feel free to email me with any questions.