fix mac test_reader_reset

Author: JiabinYang

python/paddle/fluid/tests/unittests/test_reader_reset.py

@@ -41,6 +41,8 @@ def fake_data_generator():
                 self.data_file_name, reader, feeder)
 
     def setUp(self):
+        # set parallel threads to fit 20 batches in line 49
+        os.environ['CPU_NUM'] = str(20)
         self.use_cuda = fluid.core.is_compiled_with_cuda()
         self.data_file_name = './reader_reset_test.recordio'
         self.ins_shape = [3]