Fix test cases.

First, regions have a pointer to its el. No need for a lookup. Second, I'm
detaching all contents before running the replaces. Third, never replace
top view's el, just modify.
Very. See [`#render`](https://github.com/jridgewell/minionette/blob/51718e939056df9f7352afc6a25058f6e9a9f090/src/view.js#L58-L76).

Using the "render" and "rendered" events, you can get within ~5ms of the placeholder approach I put in Test 4, all at the Dev's discretion. I didn't want to do this by default, because someone is invariably going to to to globally query while it's detached...

```javascript
view.on('render', function() {
  this._$span = Backbone.$('<span>').insertAfter(this.$el);
  this.$el.detach();
});
view.on('rendered', function() {
  this._$span.replaceWith(this.$el);
  delete this._$span;
});
```