[IMP] website, test_website: new standalone tags to ease runbot configs

As we have more and more standalone tests related to the website app, it
has been decided with the runbot team to introduce a unique tag to
easily identify those tests.

The goal is to make the runbot build config easier to manage, without
the need to add the new tag name in the command.
Instead, this build will be fired by finding and testing all the
`website_standalone` tests.
The other tags are kept to easily identify what are the test related to.

Command without this (which need to be edited for each new tag):
```
--standalone cow_views,cow_views_inherit,theme_views,theme_upgrade
```
Now:
```
--standalone website_standalone
```

Part-of: odoo/odoo#91012