Add test for a SVG to-animation (to "display:none") being sampled at underlying value.
Thanks to Robert Longson for writing an initial prototype-WPT that this WPT
is based on.
Differential Revision: https://phabricator.services.mozilla.com/D226091
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1925324
gecko-commit: b378432dac5f4dfae934158a2a34374227e1d90a
gecko-reviewers: longsonr