Random generator testing not working correctly,

[xidulu]

Description

Changing the ss.norm.ppf to ss.logistic.ppf or ss.gumbel_r.ppf will not affect the result of the test, which is clearly problematic.

https://github.com/apache/incubator-mxnet/blob/master/tests/python/unittest/test_random.py#L614

To reproduce:

1. Replace the ss.norm.ppf in line 614 in test_random.py with ss.gumbel_r.ppf
2. Run MXNET_ENGINE_TYPE=NaiveEngine nosetests -s --verbose tests/python/unittest/test_random.py:test_normal_generator

the output on my device was:

[INFO] Setting module np/mx/python random seeds, use MXNET_MODULE_SEED=1198992244 to reproduce.
[13:40:41] ../src/engine/engine.cc:55: MXNet start using engine: NaiveEngine
test_random.test_normal_generator ... ok

----------------------------------------------------------------------
Ran 1 test in 5.783s

OK
[13:40:47] ../src/engine/naive_engine.cc:72: Engine shutdown