Improved test assertions

[Jeffrey Warren]

We should be able to parse the returned data in tests to be sure they include an image; in general, tests are a little sparse; this tests did not fail although it errored:

https://travis-ci.org/publiclab/image-sequencer-app/builds/621629845?utm_source=github_status&utm_medium=notification

[Jeffrey Warren]

Here we could be parsing and running more specific assertions on the returned data:

image-sequencer-app/test/process.js

Lines 94 to 97 in ba7adc2

 .end((err, res) => { 
     res.should.be.any; 
     done(); 
 }); 

I think it should be an HTML page with an embedded image?