[SPARK-2479 (partial)][MLLIB] fix binary metrics unit tests

Allow small errors in comparison.

@dbtsai , this unit test blocks https://github.com/apache/spark/pull/1562 . I may need to merge this one first. We can change it to use the tools in https://github.com/apache/spark/pull/1425 after that PR gets merged.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1576 from mengxr/fix-binary-metrics-unit-tests and squashes the following commits:

5076a7f [Xiangrui Meng] fix binary metrics unit tests