[SPARK-8683] [BUILD] Depend on mockito-core instead of mockito-all

Spark's tests currently depend on `mockito-all`, which bundles Hamcrest and Objenesis classes. Instead, it should depend on `mockito-core`, which declares those libraries as Maven dependencies. This is necessary in order to fix a dependency conflict that leads to a NoSuchMethodError when using certain Hamcrest matchers.

See https://github.com/mockito/mockito/wiki/Declaring-mockito-dependency for more details.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7061 from JoshRosen/mockito-core-instead-of-all and squashes the following commits:

70eccbe [Josh Rosen] Depend on mockito-core instead of mockito-all.