Improved test assertions

[jywarren]

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

[jywarren]

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?