SET IDENTITY_INSERT inside INSTEAD OF INSERT trigger not working
The error isn't coming from the trigger.  It's coming from your insert statement.  

INSERT INTO PixTest(ID, TESTE) VALUES (1, 'TESTE 1')


It's getting parsed and returning an error before you ever get to the trigger.  Change it to this and it works.

INSERT INTO PixTest(TESTE) VALUES ('TESTE 1')