speed up unit test (#2847)

ml-agents/mlagents/trainers/tests/test_simple_rl.py

 SAC_CONFIG = """
     default:
         trainer: sac
-        batch_size: 32
-        buffer_size: 10240
-        buffer_init_steps: 1000
-        hidden_units: 64
+        batch_size: 8
+        buffer_size: 500
+        buffer_init_steps: 100
+        hidden_units: 16
-        max_steps: 2000
+        max_steps: 1000
         memory_size: 256
         normalize: false
         num_update: 1