Exclude useless gradle test dependency

[jeromepochat]

jenkins-test-harness-tools depends on gradle:2.15. This causes an issue when running PCT on megawar containing docker-workflow-plugin and gradle:2.16.1149.v711b_998b_0532 (see JENKINS-76087).

Let's exclude gradle which is not used from here.

Testing done

I verified locally that "mvn clean test" succeed.

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