[FIX] test_mail: fix activity widget test

Before this PR, any fail during the QUnit test suite would have failed
the activity widget test. This is due to the fact that this test was
relying on the activity table to be the only table present in the
document. This issue is that QUnit is also using table to display
failed tests. Thus, the activity widget test was looking in the wrong
table to find its elements.

This commit fixes the issue by ensuring the test is actually targeting
the right table that is, the one inside the activity view.

Part-of: odoo/odoo#99924

addons/test_mail/static/tests/activity_tests.js

@@ -263,7 +263,7 @@ QUnit.test('activity view: activity widget', async function (assert) {
         },
     });
 
-    const activity = $(document);
+    const activity = $('.o_activity_view');
     var today = activity.find('table tbody tr:first td:nth-child(2).today');
     var dropdown = today.find('.dropdown-menu.o_activity');