Flink YARN tests

flink-yarn-test collects test cases which are deployed to a local Apache Hadoop YARN cluster. There are several things to consider when running these tests locally:

• YarnTestBase spins up a MiniYARNCluster. This cluster spawns processes outside of the IDE's JVM to run the workers on. JAVA_HOME needs to be set to make this work.
• The Flink cluster within each test is deployed using the flink-dist binaries. Any changes made to the code will only take effect after rebuilding the flink-dist module.
• Each YARN*ITCase will have a local working directory for resources like logs to be stored. These working directories are located in flink-yarn-tests/target/ (see find flink-yarn-tests/target -name "*.err" -or -name "*.out" for the test's output).
• There is a known problem causing test instabilities due to our usage of Hadoop 2.8.5 executing the tests. This is caused by a bug YARN-7007 that got fixed in Hadoop 2.8.6. See FLINK-15534 for further details on the related discussion.