[SPARK-9126] [MLLIB] do not assert on time taken by Thread.sleep()

Measure lower and upper bounds for task time and use them for validation. This PR also implements `Stopwatch.toString`. This suite should finish in less than 1 second.

jkbradley pwendell

Author: Xiangrui Meng <meng@databricks.com>

Closes #7457 from mengxr/SPARK-9126 and squashes the following commits:

4b40faa [Xiangrui Meng] simplify tests
739f5bd [Xiangrui Meng] do not assert on time taken by Thread.sleep()