[FIX] web: popover can have multiple popoverClass

During the forward-port of odoo/odoo#106388 I changed the way the
popoverClass is taken into account in the `onPositioned` callback and
should have written a test. (from 16.0 branch up to current master)

This commit brings the same change (splitting and spreading the prop
value) and adds a test.

closes odoo/odoo#106724

X-original-commit: ddaffdb3bdfa823c2298fc011a9e848304b553cf
Signed-off-by: Luca Vitali <luvi@odoo.com>
Signed-off-by: Bruno Boi (boi) <boi@odoo.com>