[FIX] mail: fix non deterministic crosstab test

This PR fixes a test that ensures the bus subscriptions are updated
when a member is invited to a channel. This test waits some time
before checking if the subscription is done. Sometimes, a tick is not
enough and the test fails. This PR adds a deferred that resolves when
the subscription is done that can be awaited to ensure the timing is
always right.

fixes runbot-46941

closes odoo/odoo#146848

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/tests/crosstab/crosstab_tests.js

@@ -1,6 +1,7 @@
 /* @odoo-module */
 
 import { startServer } from "@bus/../tests/helpers/mock_python_environment";
+import { waitUntilSubscribe } from "@bus/../tests/helpers/websocket_event_deferred";
 
 import { start } from "@mail/../tests/helpers/test_utils";
 
@@ -122,7 +123,7 @@ QUnit.test("Channel subscription is renewed when channel is left", async (assert
     openDiscuss();
     await click(".o-mail-DiscussSidebarChannel .btn[title='Leave this channel']");
     await contains(".o-mail-DiscussSidebarChannel", { count: 0 });
-    await new Promise((resolve) => setTimeout(resolve)); // update of channels is debounced
+    await waitUntilSubscribe();
     assert.verifySteps(["update-channels"]);
 });