Bug 1432037 - Wait for tab animation end when closing test tabs. r=bgrins

MozReview-Commit-ID: 2XxcOFOqJo

--HG--
extra : rebase_source : ae7482dc3ebc67285f6ca666804edb2819280ca8

testing/talos/talos/tests/devtools/addon/content/damp.js

@@ -278,8 +278,13 @@ Damp.prototype = {
     });
   },
 
-  closeCurrentTab() {
+  async closeCurrentTab() {
+    let onTransition = new Promise(done => {
+      this._win.gBrowser.tabContainer.addEventListener("transitionend", done,
+        { once: true });
+    });
     this._win.BrowserCloseTabOrWindow();
+    await onTransition;
     return this._win.gBrowser.selectedTab;
   },
 
@@ -907,7 +912,7 @@ async _consoleOpenWithCachedMessagesTest() {
   },
 
   async testTeardown(url) {
-    this.closeCurrentTab();
+    await this.closeCurrentTab();
 
     // Force freeing memory now so that it doesn't happen during the next test
     await garbageCollect();