Extend gentest harness to support image nodes

[nicoburns]

What problem does this solve or what need does it fill?

Images nodes have an "intrinsic aspect ratio". And this behaves slightly differently to a node with the aspect_ratio style set, so it would be useful to be able to create gentests for it.

What solution would you like?

In Taffy's terms, a intrinsic aspect ratio corresponds to measure function which will determine the other dimension based on an aspect ratio it is asked to measure itself with one dimension set. So I would like to propose that we allow for setting image nodes in gentests, and translate that into such a measure function.

We should have 2-3 standard images to use:

• One which is square (1:1 aspect ratio)
• One which is landscape (wider than it is tall)
• One which is portrait (taller than it wide)

Additional context

The following can be used as an initial test cases:

<div id="test-root" style="display: flex; flex-direction: column; width: 200px;">
     <img src="../../scripts/gentest/3x2.png" />
</div>

<div id="test-root" style="display: flex; flex-direction: row; height: 100px;">
     <img src="../../scripts/gentest/1x1.png" />
</div>