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

Due to some timing issues, Firefox would load $anchorScroll (which in turn
would cache `$window.document` for later use), before `$window.document`
was mocked. Consequently, tests depending on `$window.document`'s being
mocked would fail.
Mocking `$window.document` during module initialization ensures that
$anchorScroll will always cache (and use) the mocked document.