NIFI-2098 Adding ignore annotation to testManyFilesOpened unit test

[JPercivall]

No description provided.

[markap14]

@JPercivall I think this unit test is actually quite helpful to have. It can add a little while to the build process, but it ensures that we don't accidentally hold open the file handles that we are not supposed to hold open. I think the System.out.println() calls need to be replaced with logger calls but don't think we should Ignore the test. If we want to avoid running it too often, we should probably look at making it an integration test instead.