Add some tests to clarify the "must-change-password" behavior (#30693)

Follow  #30472:

When a user is created by command line `./gitea admin user create`:

Old behavior before #30472: the first user (admin or non-admin) doesn't
need to change password.

Revert to the old behavior before #30472

cmd/admin_user_change_password.go

@@ -35,7 +35,7 @@ var microcmdUserChangePassword = &cli.Command{
 		},
 		&cli.BoolFlag{
 			Name:  "must-change-password",
-			Usage: "User must change password",
+			Usage: "User must change password (can be disabled by --must-change-password=false)",
 			Value: true,
 		},
 	},