NIFI-5318 Implement NiFi test harness: initial commit of nifi-test

[peter-gergely-horvath]

Thank you for submitting a contribution to Apache NiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
  Could not run this, I have test failures in my environment, that are unrelated to the code changes:

[INFO] Running org.apache.nifi.provenance.index.lucene.TestLuceneEventIndex
[ERROR] Tests run: 8, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 61.062 s <<< FAILURE! - in org.apache.nifi.provenance.index.lucene.TestLuceneEventIndex
[ERROR] testUnauthorizedEventsGetPlaceholdersForExpandChildren(org.apache.nifi.provenance.index.lucene.TestLuceneEventIndex)  Time elapsed: 60.014 s  <<< ERROR!
org.junit.runners.model.TestTimedOutException: test timed out after 60000 milliseconds
        at org.apache.nifi.provenance.index.lucene.TestLuceneEventIndex.testUnauthorizedEventsGetPlaceholdersForExpandChildren(TestLuceneEventIndex.java:206)

• Have you written or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.