[FIX] web: fix randomly failing form tests

We must wait for an additional tick to be sure that the error
dialog is displayed, because the "unhandledrejection" event is
triggered asynchronously.

Runbot issue-24690
Runbot issue-24691
Runbot issue-24744
Runbot issue-24733
Runbot issue-24742

closes odoo/odoo#135426

Signed-off-by: Pierre Rousseau (pro) <pro@odoo.com>