Make AWSY scroll during testing, or at least add some tests that scroll

[sethfowler]

Some issues only show up during scrolling, especially bugs related to image or graphics memory management code. Here's an example:

https://bugzil.la/1284350

This was a pretty serious bug that would've been caught if AWSY scrolled during the tests.

It'd be great if we could either make AWSY scroll during all tests, or at least add some tests that specifically check for issues during scrolling.

[EricRahm]

This seems reasonable, I get the feeling it will mess up our memory stats initially but that's probably okay as long as we annotate it.

I've played around with this previously, doing obvious things via keyboard shortcuts (pagedown, spacebar, etc) doesn't seem to work but using JS via window.scrollBy(0, window.innerHeight); seems to work sometimes. I think the things might get messed up for pages that steal input. Either way I'll see if I can dig up that code.