Update test_reader_reset.py

import os module to use os.environ in setUp()

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

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 from __future__ import print_function
-
+import os
 import paddle.fluid as fluid
 import paddle
 import numpy as np