Enable py4j integration tests in system-tests

There are 26 tests (+1 added now) in PythonControllerInteractionIT.java, which
is in nifi-py4j-integration-tests, but these were not run in any of the CI
jobs. I have added them to system-tests, because integration-tests explicitly
skips nifi-py4j-integration-tests with a comment that it is run as part of
system-tests.

Author: fgerlits

.github/workflows/system-tests.yml

@@ -61,6 +61,7 @@ env:
     -pl :nifi-python-framework
     -pl :nifi-python-extension-api
     -pl :nifi-python-test-extensions
+    -pl :nifi-py4j-integration-tests
     -pl nifi-system-tests/nifi-system-test-suite
     -pl nifi-system-tests/nifi-stateless-system-test-suite