Bug 1047275 - Test ellipsis side on RTL pages for FontSizeManager

Add unit tests for testing ellipsis side on RTL pages for FontSizeManager

Also refactor unit tests introduced in Bug 1047998 to prevent repeated code

https://bugzilla.mozilla.org/show_bug.cgi?id=1047275
I'm not convinced on the value of this, but since it's already there and it's testing something that we cover, we'd might as well keep it.