merge_pr_46344

Move some scrollend subtests to test variants.

Tests is scrollend-event-fired-for-programmatic-scroll.html execute
several subtests in a single promise_test call. This can make debugging
a intermittent failure a bit more difficult. Move the subtests to test
variants to hopefully make debugging intermittent failures a bit easier.

Differential Revision: https://phabricator.services.mozilla.com/D208406

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1835844
gecko-commit: 16b7bc7da3f82a38352aa97091f51b41caf89c3a
gecko-reviewers: hiro