[FIX] website: add test for onFocus/onBlur flow

The flow has been broken multiple times across the years. Let's add a
test in all stable versions. In 14.0 and 15.0, bugs in that flow have
been reintroduced in multiple ways, those will be fixed during the
forward-port of this test.

closes odoo/odoo#79579

Signed-off-by: Romain Derie (rde) <rde@odoo.com>