Fix creation of pinot-integration-tests jar

Currently the `pinot-integration-tests` jar is empty (other than a manifest), versus containing the very useful `BaseClusterIntegrationTest` class and related code. The issue is that it should be being build as a tests jar, using code from the `src/test/` directory.

In addition, it would be good to make it a fully shaded jar, so that its dependencies on other jars (for example, ZooKeeper) don't cause jar hell with project code that is using this code for integration testing.

Start by locating the build configuration for the pinot-integration-tests jar and the BaseClusterIntegrationTest code under src/test/. Check how the tests jar is assembled and how dependencies such as ZooKeeper are packaged. Done means the jar contains the integration-test classes and is shaded so its dependencies do not cause jar conflicts.

Written by the indexing model from the issue text.