fix testNG tests and reactivate task

[simonbasle]

The testNG task currently excludes all files, resulting in NO-SOURCE for the task.
If the exclude is removed and we add an includes.clear(), the correct source set is used for testNG tests, but then some tests fail on CI (see this Travis CI build).

Additional tests from `testNG` ([**/*Verification.*])


Gradle suite > Gradle test > reactor.core.publisher.tck.FluxWithProcessorVerification.mustImmediatelyPassOnOnErrorEventsReceivedFromItsUpstreamToItsDownstream FAILED

    java.lang.AssertionError


Gradle suite > Gradle test > reactor.core.publisher.tck.FluxWithProcessorVerification.required_spec209_mustBePreparedToReceiveAnOnCompleteSignalWithPrecedingRequestCall FAILED

    java.lang.AssertionError


Gradle suite > Gradle test > reactor.core.publisher.tck.FluxWithProcessorVerification.required_spec209_mustBePreparedToReceiveAnOnCompleteSignalWithoutPrecedingRequestCall FAILED

    java.lang.AssertionError


Gradle suite > Gradle test > reactor.core.publisher.tck.FluxWithProcessorVerification.required_spec210_mustBePreparedToReceiveAnOnErrorSignalWithPrecedingRequestCall FAILED

    java.lang.AssertionError


Gradle suite > Gradle test > reactor.core.publisher.tck.FluxWithProcessorVerification.required_spec210_mustBePreparedToReceiveAnOnErrorSignalWithoutPrecedingRequestCall FAILED

    java.lang.AssertionError


348 tests completed, 5 failed, 107 skipped

:reactor-core:testNG FAILED


FAILURE: Build failed with an exception.


* What went wrong:

Execution failed for task ':reactor-core:testNG'.

> There were failing tests. See the report at: file:///home/travis/build/reactor/reactor-core/reactor-core/build/reports/tests/testNG/index.html