[bug-fix] Increase buffer size for SAC tests (#3813)

Author: Ervin T

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

@@ -56,7 +56,7 @@
     {BRAIN_NAME}:
         trainer: sac
         batch_size: 8
-        buffer_size: 500
+        buffer_size: 5000
         buffer_init_steps: 100
         hidden_units: 16
         init_entcoef: 0.01