SQL Server 2014 Database vs. Schema Permissions
I'd like to grant create table to the test schema for a windows group;
  but not have them be able to create a table in the dbo schema.


The correct grants are:

GRANT ALTER ON SCHEMA::test TO [ad\windows_group];

GRANT CREATE TABLE TO [ad\windows_group];



  but not have them be able to create a table in the dbo schema.


They would not be able to do that without ALTER permissions on dbo.  


  I think I'm missing something.


Somehow you've granted that user ALTER on the dbo schema or the database.