Test that no clamping occurs before interpolation

In the context of color, the spec encourages late clamping but does not
require it:
"User agents should clamp color values to allow color range values as
late as possible, but note that system differences might preclude
consistent behavior across different systems."
http://www.w3.org/TR/SVG/animate.html#AnimateColorElement

With opacity, nothing prevents us clamping late, after interpolation.
Use of late clamping has been observed on the public web.
Opacity should be tested as two static rectangles next to each other, one for polyfill and one for native.