Multisite: Handle get_id_from_blogname() lookups when the network domain has www..
Previously, if a network's domain started with www. in a subdomain configuration, a slug lookup with get_id_from_blogname() would not match an existing site. A similar lookup in a subdirectory configuration would work fine.
This strips www. from the network's domain in a subdomain configuration during the lookup and returns the site as expected.
Adds tests which would previously fail in a subdomain configuration, but now pass in both configurations.
Props igmoweb, flixos90.
Fixes #34450.