"User name" field no longer selected by default on login screen

[AdamWill]

Up until cockpit 118, when you land on the cockpit login screen, the "User name" field is pre-selected and you can just start typing the user name. With cockpit 118 (in Fedora Rawhide), it is not: you have to click the field or hit tab before you can type the user name.

[AdamWill]

looks like #4991 fixed this already.

[stefwalter]

Thanks @AdamWill and congratulations on #5000 :)

I also added a regression test for this, making sure the expected behavior sticks.