[SPARK-9454] Change LDASuite tests to use vector comparisons

jkbradley Changes the current hacky string-comparison for vector compares.

Author: Feynman Liang <fliang@databricks.com>

Closes #7775 from feynmanliang/SPARK-9454-ldasuite-vector-compare and squashes the following commits:

bd91a82 [Feynman Liang] Remove println
905c76e [Feynman Liang] Fix string compare in distributed EM
2f24c13 [Feynman Liang] Improve LDASuite tests