[FIX] mail: adapt "history" test with non deterministic issue

Follow up on 255f7219

The issue is still happening.

With the current commit, we will be able to know for sure if it happens because
we don't wait long enough, or because the messages are never appearing.

closes odoo/odoo#44471

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/tests/discuss_tests.js

@@ -1913,6 +1913,14 @@ QUnit.test('all messages in "Inbox" in "History" after marked all as read', asyn
     await testUtils.nextTick();
     assert.verifySteps(['message_fetch'],
         "should fetch messages once for history");
+    let hasEnoughMessages = false;
+    while (!hasEnoughMessages) {
+        if (discuss.$('.o_thread_message').length === 30) {
+            hasEnoughMessages = true;
+        } else {
+            await testUtils.nextTick();
+        }
+    }
     assert.containsN(discuss, '.o_thread_message', 30,
         "there should be 30 messages in History");