Export mockito in gerrit-test-util
Export mockito in gerrit-test-util

This is needed to be able to use mockito in plugin tests.

Easymock was removed in change Ie01e53747 but adding mockito in
its place was overlooked.

Bug: Issue 5057
Change-Id: I5e3f35d1d9328d9bcb5babe59ead3ae6ca730701

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