[IMP] tests, *: add start_tour helper

Before this commit, the syntax to start a tour was extremely verbose.
With this new method, it is possible to start a tour by just giving the
essential parameter: the tour name.
The full set of features from browser_js are kept by using **kwargs.

closes odoo/odoo#32316

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>