Fix race condition in test

sky/tests/editing/delete_block_contents.sky

 <sky>
 <import src="../resources/chai.sky" />
 <import src="../resources/mocha.sky" />
 This test verifies that the height of an editable block remains the same after adding block elements and removing them.
 <div contenteditable="true" style="border: solid blue" id="test"></div>
 <script>
-describe("height of an editable block", function(done) {
-  it("remains the same after adding block elements and removing them", function() {
+describe("height of an editable block", function() {
+  it("remains the same after adding block elements and removing them", function(done) {
     var elem = document.getElementById("test");
     var originalHeight = elem.offsetHeight;
     var d = elem.appendChild(document.createElement('div'));
     d.appendChild(document.createTextNode('aaa'));
     d = elem.appendChild(document.createElement('div'));
     d.appendChild(document.createTextNode('bbb'));
     var newHeight = elem.offsetHeight;
 
     while (elem.firstChild) {
        elem.removeChild(elem.firstChild);
     }
 
     setTimeout(function() {
       assert.equal(elem.offsetHeight, originalHeight);
       done();
     });
   });
 })
 </script>
 </sky>