Get rid of cyclic tests (border-radius tests) about csswg-test

[gsnedders]

Unless I'm being stupid, the following should match (top-left):

<svg width="200" height="100"
     xmlns="http://www.w3.org/2000/svg">
 <path d="M25,0 a25,25 0 0,0 -25,25 V100 H200 V0 z"
        fill="#dddddd" stroke="#a1a1a1" stroke-width="2" />
</svg>

[gsnedders]

Oh, using SVGs for refs for border-radius falls into the trap of SVG strokes being centred on the edge of the path and CSS borders being outside the path, which also makes drawing the border arc impossible, given in CSS the arc radii are those on the outside edge of the border.

Any ideas, @tabatkins?

[gsnedders]

I mean I guess we can test it if we have fill/stroke & background/border the same colour, but that isn't great. Maybe the best we can do with references, though?

[tabatkins]

The simple case is to make the stroke double-width, and then put the same path on top of it with an opaque fill and no stroke. I think that'll give the same geometry as a border stroke outside of the box.

[wpt-issue-mover]

This issue has been moved to web-platform-tests/wpt#5492; please continue all discussion there.