Testing related realms

Related realms is feature to provide login suggestion based on similar domains.

Out of the scope of this feature is subdomain handling, which is already part
of the base functionality. The intent is to cover the ebay.com/ebay.co.uk and
all other country TLD cases where the sign in page is actually
signin.ebay.com/signin.ebay.co.uk but credentials could have manually been
entered for ebay.com/ebay.co.uk or automatically stored as
signin.ebay.com/sigin.ebay.co.uk

The related realms feature can be enabled via the preference
signon.relatedRealms.enabled. It is disabled by default.

This test proves that related logins show up in the autocomplete menu when
the feature is enabled and that the related logins do not count as duplicates
and therefore the form gets filled directly.