Since these tests concern the functionality between Charts and Layers, I think they'd fit better in the `integration.js` test file. Maybe we should re-name that `interaction.js`?

These tests deserve dedicated suites (`#unlayer` and `#relayer`), not just for consistency, but to completely exercise the functionality.

For instance:

```js
test("disjoins child from parent", function() {
	this.chart.unlayer("testLayer");
	assert.isUndefined(this.layer.chart());
});
```

...and later:

```js
test("joins child to parent", function() {
    var newLayer = this.base.append("g").classed("layer2", true).layer();
    this.chart.relayer("newLayer", newLayer);
    assert.equal(this.chart.layer("newLayer"), newLayer);
});
```