[SPARK-8709] Exclude hadoop-client's mockito-all dependency

This patch excludes `hadoop-client`'s dependency on `mockito-all`.  As of #7061, Spark depends on `mockito-core` instead of `mockito-all`, so the dependency from Hadoop was leading to test compilation failures for some of the Hadoop 2 SBT builds.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7090 from JoshRosen/SPARK-8709 and squashes the following commits:

e190122 [Josh Rosen] [SPARK-8709] Exclude hadoop-client's mockito-all dependency.