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'd suggest refactoring this into a function like this:
```js
function getExpectedTextContent(side, direction) {
  return side == 'left' ^ direction == 'ltr' ? '\u2026ar' : 'fo\u2026';
}
```