[FIX] web: attempt to fix non deterministic issue in tests

This commit *could* prevent a test from failing randomly. The
important part in the diff is the switch from $.Deferred to native
Promise. JQuery Deferred behave differently upon resolution (it is
sync if the deferred is resolved directly, whereas it is not the
case for native Promise). I couldn't find any other reason to
explain why the test was sometimes failing. There is no reason to
use Deferred anyway.

The issue didn't occur after this commit in more than 600 builds.

[1] https://runbot.odoo.com/web#id=2297&action=573&model=runbot.build.error&view_type=form&cids=1&menu_id=405

Part-of: odoo/odoo#78621