fix($anchorScrollSpec): fix tests randimly failing in Firefox

Due to some timing issue, Firefox would load $anchorScroll (which in turn
would cache `$window.document` for later use), before `$window.document`
was mocked (for the tests that needed this.
Mocking `$window.document` for every test during module initialization
ensured that $anchorScroll will always cache (and use) the mocked
document.