Write tests for autosizes
-measure (see the parent task) parser with Mocha, so they are run and understood by Phabricator.
Include 5 test cases:
1. A trivial one
```
<!DOCTYPE html>
<html>
<head></head>
<body>
<div style="background-color: red; height: 20px; width: 100vw"></div>
</body>
</html>
```
(measure the div element width)
2. A responsive grid of images that has more columns when there's more horizontal space
3. A single image that does not change size no matter the viewport width
4. A single image that appears in the DOM after 2000ms artificial delay and has constant size
5. A page that does not contain the target element
They should be local HTML files with embedded CSS that the tool will be using for measurement. Probably they can just be passed as a file:// url argument