Add test of JSON output

[rouson]

Sample output:

{
	"dag": {
		"vertices": [{
				"edges": [],
				"checking": false,
				"marked": false,
				"label": "",
				"attributes": ""
			},
			{
				"edges": [1],
				"checking": false,
				"marked": false,
				"label": "foo",
				"attributes": "bar"
			}
		]
	}
}