Fix test-compositing-order.html
Fix test-compositing-order.html

This intent of the test was broken by 1e21b9ee86fb7225494257a787dc9e0fbb5a01a5
though the test continued to pass.

dstockwell/web-animations-js,dstockwell/web-animations-js,birtles/web-animations-js,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy