You should probably write your test like this:
it('tests this directive', inject(function($rootScope, $compile){
var scope = $rootScope.$new();
scope.myModelValue = 'something';
var html = '<input format ng-model="myModelValue"></input>'
var element = $compile(html)(scope);
// I can't remember, but I think you need to call a scope.$apply() here
// element.val() will return formatted value from the $parsers
// Then element.val('something else') && scope.$apply() should change myModelValue from $formatters
}));