chore(test): use flush function instead of custom one

Before this commit some unit test suites were using custom function to
delay assertions because of the latency of the DOM to be updated by
dom-repeat and dom-if. Now, they are using the function "flush" (that is
provided by web-component-teser?).

See: https://www.polymer-project.org/2.0/docs/tools/tests#hybrid-elements-test-local-dom

test/login-fire-social_test.html

@@ -26,24 +26,20 @@
   </test-fixture>
 
   <script>
-    /**
-     * Adds delay to a function call. Used for assertions after templates "dom-if"
-     * and "dom-repeat" restamping. Seems those tempaltes need time for re-redering.
-     */
-    const afterNextRender = (f, delay = 5) => setTimeout(f, delay);
-
     describe('Behavior Audit - Fixture: Default', function() {
       let social;
 
-      beforeEach(function(done) {
+      beforeEach(function() {
         social = fixture('Default');
-        afterNextRender(done);
       });
 
-      it('Should display only "Anonymous" button', function() {
-        let buttons = (social.shadowRoot || social).querySelectorAll('paper-social-button');
-        expect(typeof buttons).to.be.Array;
-        expect(buttons.length).to.be.equal(1);
+      it('Should display only "Anonymous" button', function(done) {
+        flush(function() {
+          let buttons = (social.shadowRoot || social).querySelectorAll('paper-social-button');
+          expect(typeof buttons).to.be.Array;
+          expect(buttons.length).to.be.equal(1);
+          done();
+        });
       });
 
       it('Should have only "Anonymous" provider in its list', function() {
@@ -69,11 +65,9 @@
 
       it('Should sign-in with the provider associated to the clicked button', function(done) {
         social.providers = 'facebook, google, twitter';
-        afterNextRender(() => {
+        sinon.stub(social, 'signInOrOut');
+        flush(function() {
           const button = (social.shadowRoot || social).querySelectorAll('paper-social-button')[1];
-          stub('login-fire-social', {
-            signInOrOut: null,
-          });
           button.addEventListener('click', function(event) {
             expect(social.provider).to.be.equal('google');
             expect(social.provider).to.be.equal(event.target.provider.id);
@@ -85,15 +79,15 @@
 
       it('Should display only a sign-out button when user is signed-in', function(done) {
         social.addEventListener('signedin', function() {
-          afterNextRender(() => {
+          flush(function() {
             expect(social.signedIn).to.be.true;
             let buttons = (social.shadowRoot || social).querySelectorAll('paper-social-button');
             expect(buttons.length).to.be.equal(1);
             expect(buttons[0]._text).to.contain('Sign out');
             done();
           });
         });
-        (social.shadowRoot || social).querySelector('paper-social-button').click();
+        social.signInWith('anonymous');
       });
 
       it('Should apply attribute "flat" on all buttons', function() {