[FIX] website: CSS properties value comparison

Due to a change between Chrome 101 and 114, the CSS Custom Property
containing custom string are now always serialized using the type of
quotes used in their declaration (ie. single quotes will be kept ;
double quotes also) which wasn't the case in previous versions of Chrome
(and still not the case in WebKitGTK 2.40.0 at least).

This doesn't impact the value itself but only comparison between the
computed value and an arbitrary one (like we do in the design-themes
test).

This commit fixes it by removing the single/double quotes in the
`assertCssVariable` test helper.

The impacted precondition was introduced in odoo/odoo@b29e17765f4e912b2dd472493b5be500b3a32c87 and
odoo/design-themes@8b377af3a71b875dfc452a14206a9a1679371000 .

closes odoo/odoo#125404

Signed-off-by: Romain Derie (rde) <rde@odoo.com>