Add test for a random floating-point number to make sure we create the transition for the custom properties.
Differential Revision: https://phabricator.services.mozilla.com/D223629
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1917331
gecko-commit: c969e2c6952e6bd4b9f7f1295f130d45d1377a61
gecko-reviewers: layout-reviewers, firefox-style-system-reviewers, emilio