AngularJS template in directive not display text - angularjs

Question

I try to create directive in angularjs:
JavaScript and HTML code:
'use strict';
var app = angular.module('myApp', ['app.directives']);
app.controller('AppCtrl', function($scope){
$scope.roles = [
{"id": 1, "name": "Michael"},
{"id": 2, "name": "Max"},
{"id": 3, "name": "John"}
];
$scope.dummyData = "Dummy!!!";
$scope.member = {roles: []};
$scope.selected_items = [];
});
var app_directives = angular.module('app.directives', []);
app_directives.directive('dropdownMultiselect', function(){
return {
restrict: 'E',
scope:{
testElem: '=',
model: '=',
options: '=',
pre_selected: '=preSelected'
},
template: "<div class='btn-group' data-ng-class='{open: open}'>"+
"<button class='btn btn-small'>{{testElem}}</button>"+
"<button class='btn btn-small dropdown-toggle' data-ng-click='open=!open;openDropdown()'><span class='caret'></span></button>"+
"<ul class='dropdown-menu' aria-labelledby='dropdownMenu'>" +
"<li><a data-ng-click='selectAll()'><i class='icon-ok-sign'></i> Check All</a></li>" +
"<li><a data-ng-click='deselectAll();'><i class='icon-remove-sign'></i> Uncheck All</a></li>" +
"<li class='divider'></li>" +
"<li data-ng-repeat='option in options'> <a data-ng-click='setSelectedItem()'>{{option.name}}<span data-ng-class='isChecked(option.id)'></span></a></li>" +
"</ul>" +
"</div>" ,
controller: function($scope){
$scope.openDropdown = function(){
$scope.selected_items = [];
for(var i=0; i<$scope.pre_selected.length; i++){ $scope.selected_items.push($scope.pre_selected[i].id);
}
};
$scope.selectAll = function () {
$scope.model = _.pluck($scope.options, 'id');
console.log($scope.model);
};
$scope.deselectAll = function() {
$scope.model=[];
console.log($scope.model);
};
$scope.setSelectedItem = function(){
var id = this.option.id;
if (_.contains($scope.model, id)) {
$scope.model = _.without($scope.model, id);
} else {
$scope.model.push(id);
}
console.log($scope.model);
return false;
};
$scope.isChecked = function (id) {
if (_.contains($scope.model, id)) {
return 'icon-ok pull-right';
}
return false;
};
}
}
});
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.3.0/css/bootstrap-combined.min.css" rel="stylesheet"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.js"></script>
<link href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<div ng-app="myApp" ng-controller="AppCtrl">
<dropdown-multiselect pre-selected="member.roles" model="selected_items" testElem = "dummyData" options="roles"></dropdown-multiselect>
In template in the directive definition I have this row:
<button class='btn btn-small'>{{$scope.testElem}}</button>
As you can see with help of this row I try to display text inside $scope.testElem variable.
But the text does not appears.What I am missing? Why I don't see text on the declared button?