[tests] Increase buffer_init_steps for recurrent sac test (#4051)

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

         SAC_CONFIG.hyperparameters,
         batch_size=128,
         learning_rate=1e-3,
-        buffer_init_steps=500,
+        buffer_init_steps=1000,
         steps_per_update=2,
     )
     config = attr.evolve(