Decrease success rate to make test more stable (apache#12092)

I have added this test back to unit test coverage and decreased success rate even more, to make sure that fails would happen even more rare

tests/python/unittest/test_random.py

@@ -447,20 +447,20 @@ def test_uniform_generator():
  verify_generator(generator=generator_mx_same_seed, buckets=buckets, probs=probs)
 
 @with_seed()
-@unittest.skip('Flaky test, tracked in: https://github.com/apache/incubator-mxnet/issues/9856')
 def test_gamma_generator():
+ success_rate = 0.05
  ctx = mx.context.current_context()
  for dtype in ['float16', 'float32', 'float64']:
  for kappa, theta in [(0.5, 1.0), (1.0, 5.0)]:
  print("ctx=%s, dtype=%s, Shape=%g, Scale=%g:" % (ctx, dtype, kappa, theta))
  buckets, probs = gen_buckets_probs_with_ppf(lambda x: ss.gamma.ppf(x, a=kappa, loc=0, scale=theta), 5)
  generator_mx = lambda x: mx.nd.random.gamma(kappa, theta, shape=x, ctx=ctx, dtype=dtype).asnumpy()
- verify_generator(generator=generator_mx, buckets=buckets, probs=probs)
+ verify_generator(generator=generator_mx, buckets=buckets, probs=probs, success_rate=success_rate)
  generator_mx_same_seed = \
  lambda x: np.concatenate(
  [mx.nd.random.gamma(kappa, theta, shape=x // 10, ctx=ctx, dtype=dtype).asnumpy()
  for _ in range(10)])
- verify_generator(generator=generator_mx_same_seed, buckets=buckets, probs=probs)
+ verify_generator(generator=generator_mx_same_seed, buckets=buckets, probs=probs, success_rate=success_rate)
 
 @with_seed()
 def test_exponential_generator():