Exclude useless gradle test dependency (fix typo)

[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).

Follow-up of #365

This fixes a typo (the exclusion was ineffective).

Testing done

mvn dependency:tree | grep gradle has empty result.

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