test_mousescroll.xul becomes permanent orange due to the animation at handling wheel event. Therefore, this patch rewrites it with generators.
Note that this removes |delta(X|Y) == 0| cases from testArrowScrollbox() because it doesn't cause wheel events, therefore, it's impossible to test without timer and it's not worthwhile to test the cases that no wheel events are fired.
Additionally, this adds todo to testArrowScrollBox() because |scrollPosition += px| in <scrollbox>.scrollByPixels doesn't work fine only on macOS with this test. Even when the px is larger than its scrollSize, the result isn't set to scrollSize. This issue shouldn't be the scope of bug 1320609, therefore, this patch makes it todo. (Of course, I don't see any problem with actual UI such as tabbar and bookmark menus.)
MozReview-Commit-ID: AmAf1YmgkBn
cfbffa58c7