testing.html improvements

This branch improves the `testing.html` file in how its tests work. Before, you would write a file like
```
// svg, data, Plottable are give
var x = new Plottable....();
x.renderTo(svg);
```
However, this had a couple of problems:
- you're passed only an `svg`, so you can't have buttons
- `data` is the same for all tests, so it's always the result of `makeRandomData(50)`
- the scoping is weird

Now when you want a test, you write
```
function makeData() {
  return makeRandomData(50);
}

function run(div, data, Plottable) {
  var svg = div.append("svg").attr("width", 500);
  ...
  x.renderTo(svg);
}
```

This should be how quicktests that you want to share with other people should go. The old `quicktests/*.html` are removed.