[SPARK-7432] [MLLIB] fix flaky CrossValidator doctest
[SPARK-7432] [MLLIB] fix flaky CrossValidator doctest

The new test uses CV to compare `maxIter=0` and `maxIter=1`, and validate on the evaluation result. jkbradley

Author: Xiangrui Meng <9616cf314f60cbadfd3d5d51110130ad7923defb@databricks.com>

Closes #6572 from mengxr/SPARK-7432 and squashes the following commits:

c236bb8 [Xiangrui Meng] fix flacky cv doctest

(cherry picked from commit bd97840d5ccc3f0bfde1e5cfc7abeac9681997ab)
Signed-off-by: Xiangrui Meng <9616cf314f60cbadfd3d5d51110130ad7923defb@databricks.com>

andrewor14/iolap,andrewor14/iolap,andrewor14/iolap,andrewor14/iolap,andrewor14/iolap,andrewor14/iolap,andrewor14/iolap