Writing Unit test for Password Matching directive

I have a directive here I'm trying to write a unit test for - first time doing this type of thing. I'm not sure how to go about it. Here's the directive code and HTML :

app.directive('passwordMatch', [function () {
    return {
        restrict: 'A',
        scope:true,
        require: 'ngModel',
        link: function (scope, elem, attrs, control) {
            var checker = function () {
                var e1 = scope.$eval(attrs.ngModel);
                var e2 = scope.$eval(attrs.passwordMatch);
                if(e2!=null)
                return e1 == e2;
            };
            scope.$watch(checker, function (n) {
                control.$setValidity("passwordNoMatch", n);
            });
        }
    };
}]);

<form name="signupForm">
<div class="form-group">
                
            <div class="col-sm-7">
                <span class="block input-icon input-icon-right">
            <input type="password" class="register" name="password" placeholder="Password" ng-model="signup.password" required/>
                        </span>
            </div>
        </div>
        <div class="form-group">  
            <div class="col-sm-7">
                <span class="block input-icon input-icon-right">
                           <input type="password" class="register" name="password2" placeholder="Confirm Password" ng-model="signup.password2" password-match="signup.password" required/>                                
                           <small class="errorMessage" data-ng-show="signupForm.password2.$dirty && signupForm.password2.$error.passwordNoMatch && !signupForm.password2.$error.required"> Password do not match.</small>
                        </span>
            </div>
        </div>
</form>

And here's what I'm trying for a test. So for, it is giving me an error reading TypeError: 'undefined' is not an object (evaluating 'scope.signup.password = '123'')

describe('passwordMatch Directive - ', function() {
    
    var scope, $compile, $window, element, elm;
       
    beforeEach(function() {
      module('myApp');
      inject(function(_$compile_, _$rootScope_, _$window_) {
        $compile = _$compile_;
        scope = _$rootScope_.$new();
        $window = _$window_;
      })
      elm 
    })

    it('should indicate invalid when the passwords do not match.', function() {
        scope.signup.password = '123';
        scope.signup.password2 = '1234';
        element = $compile(angular.element('<input type="password" class="register" name="password" placeholder="Password" ng-model="signup.password" required/> <input type="password" class="register" name="password2" placeholder="Confirm Password" ng-model="signup.password2" password-match="signup.password" required/>'))(scope);
        scope.$apply();
        console.debug('element html - ' + element.html());
        expect(element.html().indexOf('ng-invalid')).toBeGreaterThan(0);
    });

    it('should indicate valid when the passwords do not match.', function() {
        scope.signup.password = '123';
        scope.signup.password2 = '123';
        element = $compile(angular.element('<input type="password" class="register" name="password" placeholder="Password" ng-model="signup.password" required/> <input type="password" class="register" name="password2" placeholder="Confirm Password" ng-model="signup.password2" password-match="signup.password" required/>'))(scope);
        scope.$apply();
        console.debug('element html - ' + element.html());
        expect(element.html().indexOf('ng-valid')).toBeGreaterThan(0);
    });
});

Some help would be greatly appreciated