Fix poca test

ml-agents/mlagents/trainers/tests/torch/test_poca.py

     )
     for key, val in value_estimates.items():
         assert type(key) is str
-        assert len(val) == 15
+        assert len(val) == time_horizon
-        assert len(val) == 15
+        assert len(val) == time_horizon
-        assert len(value_memories) == 15
-        assert len(baseline_memories) == 15
+        assert len(value_memories) == time_horizon
+        assert len(baseline_memories) == time_horizon
 
     (
         value_estimates,