The image made the test flaky when running on my machine, so this
doesn't seem safe at the moment. We can just hardcode the dimensions.
Eventually we should make it possible to use external images in tests,
but for now let's not flake up the CI.
According to CSS Inline Layout Module Level 3 § 2.2 Step 1. atomic
inlines should be layed out in a line box based on their margin box.
However, up until this patch we were unconditionally considering only
the border box during line box height calculation. This made us
essentially drop all vertical margins for atomic inlines.