NIFI-6184: Removed unnecessary test-jar dep from nifi-livy-processors…

[mattyb149]

… causing race condition, added assertion to catch failures

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[mattyb149]

There seems to be a race condition between the publishing of the nifi-standard-processors test-jar artifact, and the resolution by the nifi-livy-processors module. These events seem (at least lately) to be occurring in parallel, causing most/all Travis CI builds to timeout. Note that the same race condition could happen with any other parallel goal execution, if the other module's goal includes resolving the nifi-standard-processors test-jar dependency. We may want to update the POM and/or .travis.yml to ensure that artifact is published before other NAR modules are built, but in the meantime, this PR seems to stabilize the build.