GEODE-5987: Correct issue with powermock leaking JDK11-incompatible d…

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…ependencies.

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This PR is addressing the recent Concourse failure.

It appears that:

• powermock was leaking its dependencies
• Mockito as provided by this version of powermock is not JDK11 compatabile
• powermock is not actually necessary for unit-level testing and may be removed here.
• Consequently, the actual dependencies should be specified instead.

All the more reason for us to be diligent in keeping our footprint small not leaking dependencies we do not intend to be shared downstream.

Noting that precheckin does not yet test JDK11: locally tested with command

JAVA_HOME="" ./gradlew geode-rebalancer:test --tests AutoBalancerJUnitTest -PtestJVM=/path/to/.jenv/versions/11.0 -PforceTest=true

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[Comment bump to bump the Concourse dunit bumpity bump.]