Following the introduction of the `showWeekNumbers` prop on
DateTimePicker component (see commit 7552284e78),
the number of column (CSS custom property) wasn't properly defined in
the template.
This commit fixes it and adds related asserts in QUnit tests.
closes odoo/odoo#135427
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>