Bug 1761749 - In extensions installation tests, wait for about:blank to load. r=#thunderbird-reviewers,aleca

Differential Revision: https://phabricator.services.mozilla.com/D144774

--HG--
extra : rebase_source : 2ce6733d85f120c4d251a891650d2ace2dd8ddff
extra : amend_source : aa0b72c1bd9724ce8df6c2252b4933cbd21cb8d7

mail/base/test/webextensions/head.js

@@ -343,7 +343,10 @@ async function testInstallMethod(installFn, telemetryBase) {
   registerCleanupFunction(() => PermissionTestUtils.remove(testURI, "install"));
 
   async function runOnce(filename, cancel) {
-    openContentTab("about:blank");
+    let tab = openContentTab("about:blank");
+    if (tab.browser.webProgress.isLoadingDocument) {
+      await BrowserTestUtils.browserLoaded(tab.browser);
+    }
 
     let installPromise = new Promise(resolve => {
       let listener = {