[IMP] website: make test works with more website than the data ones

Those tests were raising an assert error if the DB has other websites than the
data/demo ones.

As we have a new module in Odoo 13.0 (odoo/design-themes#178)
which is creating 25 websites, and since the design-themes runbot is now
running tests, we were having a red design-themes runbot.

closes odoo/odoo#39140

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>