OS X 10.10.5 - root user suddenly activated
The "Shared" folder is - depending on your system - automatically created or created by the system after establishing a second user account on your Mac (in your case probably the test user). Its purpose is the sharing of files between several user accounts.

Those files (and folders) may be personal data, Application Support files for all users or registration/licensing data.

Therefore the read and write bits are set for all users (BTW t is a sticky bit): 

drwxrwxrwt   9 root   wheel    306B 16 Aug 22:30 Shared


The user folder of the root account is /var/root and contains the same default folders as a normal account.



After enabling the root user, the "new" user "Others" will be displayed in the login screen. This was done a "few months ago" as mentioned in your first EDIT section. There is no relation to the Update 10.10.5. You probably have done it yourself in the past or someone else who has admin access to your Mac.

The reason to be able to use your standard password with su is the identity of your password and the root password. If you change the password for root, you would have to enter this instead. If you then would like to su your admin user without knowing the root password, you would have to enter sudo su and your admin's password.