[IMP] website: test unsplash beacon

This commit ensures that the unsplash beacon calls home when an unsplash
image appears on a page.

To achieve this it patches the RPC call when the test URL contains the
test name as parameter. The patch cancels the actual beacon call to
avoid polluting data during the test, but marks the image as having had
its beacon message sent. The test then simply checks if this marker
appears on the image.

task-3360109

closes odoo/odoo#126370

X-original-commit: 1b9e8103
Signed-off-by: Outagant Mehdi (mou) <mou@odoo.com>
Signed-off-by: Benoit Socias (bso) <bso@odoo.com>