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
Having a suite with a test inside it by the same name is awkward. I would suggest changing this test description to 'sets correct line heights.'