Bug 1929105 follow-up - Fix test tripping over horrible IMAP code. r=tobyp

Differential Revision: https://phabricator.services.mozilla.com/D229067

--HG--
extra : moz-landing-system : lando

mail/base/test/browser/browser_reloadBadMessage.js

@@ -104,4 +104,8 @@ add_task(async function () {
 
     mboxSize = imapFolder.filePath.fileSize;
   }
+
+  // Clear the selection so the I/O service doesn't complain when we remove
+  // the account.
+  about3Pane.threadTree.selectedIndex = -1;
 });