Bug 1768351, add a special case for the one case that intermittently fails on windows, r=mhowell

Depends on D146727

Differential Revision: https://phabricator.services.mozilla.com/D146728

uriloader/exthandler/tests/mochitest/browser_save_filenames.js

@@ -263,7 +263,7 @@ add_task(async function save_document() {
     if (idx == 66 && AppConstants.platform == "win") {
       // This is special-cased on Windows. The default filename will be used, since
       // the filename is invalid, but since the previous test file has the same issue,
-      // this second file will be saved with a number suffix added to it. -->
+      // this second file will be saved with a number suffix added to it.
       filename = "index_002";
     }
 
@@ -720,6 +720,13 @@ add_task(async function save_download_links() {
           " was saved with the correct name when link has download attribute"
       );
     } else {
+      if (idx == 66 && filename == "index(1)") {
+        // Sometimes, the previous test's file still exists or wasn't created in time
+        // and a non-duplicated name is created. Allow this rather than figuring out
+        // how to avoid it since it doesn't affect what is being tested here.
+        filename = "index";
+      }
+
       is(
         filename,
         downloads[idx].filename,