[SPARK-19464][BUILD][HOTFIX][TEST-HADOOP2.6] Add back mockito test dep in YARN module, as it ends up being required in a Maven build

Add back mockito test dep in YARN module, as it ends up being required in a Maven build

## How was this patch tested?

PR builder again, but also a local `mvn` run using the command that the broken Jenkins job uses

Author: Sean Owen <sowen@cloudera.com>

Closes #16853 from srowen/SPARK-19464.2.