Call after methods in axe.runVirtualRule

Because `axe.runVirtualRule` isn't calling `after` methods, it can't give correct results for rules that use them. I'm not aware of anyone who's using runVirtualRule using it to run rules with an after method. It would've been reported otherwise, but someone might.

This should result in a violation:

```js
var node = new axe.SerialVirtualNode({ nodeName: 'div'});
node.children = [
new axe.SerialVirtualNode({ nodeName: 'main'}),
new axe.SerialVirtualNode({ nodeName: 'main'})
];
var results = axe.runVirtualRule('landmark-no-duplicate-main', node);
```

Start by running the supplied reproduction using axe.SerialVirtualNode and axe.runVirtualRule for landmark-no-duplicate-main. Read the runVirtualRule entry point and trace how rules' after methods are handled. Done means the example reports a violation and regression coverage verifies after-method behavior; the issue names no files or tests.

Written by the indexing model from the issue text.