Bug 1719908 follow-up - Fix error message check for beta. r=john.bieling

A different error message is used depending on the channel: https://hg.mozilla.org/mozilla-central/rev/03939cd2d0ab#l5.1

Differential Revision: https://phabricator.services.mozilla.com/D119718

--HG--
extra : amend_source : 12ef8c28efc9896d539c514423f356ce44fee9df

mail/components/extensions/test/browser/browser_ext_compose_attachments.js

@@ -239,14 +239,14 @@ add_task(async function test_without_permission() {
       // Try to use onAttachmentAdded.
       await browser.test.assertThrows(
         () => browser.compose.onAttachmentAdded.addListener(),
-        `can't access property "addListener", browser.compose.onAttachmentAdded is undefined`,
+        /browser\.compose\.onAttachmentAdded is undefined/,
         "Should reject listener without proper permission"
       );
 
       // Try to use onAttachmentRemoved.
       await browser.test.assertThrows(
         () => browser.compose.onAttachmentRemoved.addListener(),
-        `can't access property "addListener", browser.compose.onAttachmentRemoved is undefined`,
+        /browser\.compose\.onAttachmentRemoved is undefined/,
         "Should reject listener without proper permission"
       );