However, if the visitor first visits test.simoahava.com, and then moves to www.simoahava.com, these two domains will have different _ga cookies, and thus different Client IDs, and thus the user will be a different user with a new session!
The answer is in the cookieDomain setting. When you set cookieDomain to auto, the following will happen with a (fictional) domain like www.simoahava.co.uk:
GA tries to write the cookie on .co.uk, which is the first possible root domain candidate. This fails because the browser is not authorized to write a cookie on a top-level domain like that.
Next, GA tries to write the cookie on .simoahava.co.uk, which is the next possible root domain candidate. This works because that’s a valid domain to write the cookie on.