nixos/tests/user-account: add static uid for alice

A lot of tests assume that the alice user
will have a uid of 1000. Let's make that
a guarantee and be able to reference this value.

nixos/tests/common/user-account.nix

···
     { isNormalUser = true;
       description = "Alice Foobar";
       password = "foobar";
+      uid = 1000;
     };
 
   users.users.bob =