[css-shapes-1] `shape-image-threshold` lacks examples, tests

[svgeesus]

Choosing Image Pixels: the shape-image-threshold property has no examples, and also no WPT tests

[svgeesus]

Ah, I see there are four tests just hidden as shape-outside tests

[astearns]

This example mentions and uses shape-image-threshold: http://w3c.github.io/csswg-drafts/css-shapes-1/#example-12b6dfcf