[FLINK-12102][tests] Fix FlinkILoopTest on Java 9

[zentol]

What is the purpose of the change

Fixes an issue in the FlinkILoopTest where the test failed on Java 9.

From my understanding, powermock loads classes in a separate classloader during mocking. However powermock does not account for Java 9 modules (see powermock/powermock#864), due to which the loading will fail if the class is not available by default on java 9, as is the case for javax classes.

Apparently the PowerMockIgnore annotation stops powermock from separately loading the class, and thus the test can pass.

Verifying this change

I ran the test successfully on java 9 on a dev branch: https://travis-ci.org/zentol/flink/jobs/518712231

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[tillrohrmann]

Thanks for the fix @zentol. +1 for merging.