Add Testing Operator - Apache Flink

The [platform-flink](https://github.com/apache/incubator-wayang/tree/main/wayang-platforms/wayang-flink/code) was added and had just the integrations test, but it does not have the operator test like in [platform-spark](https://github.com/apache/incubator-wayang/tree/main/wayang-platforms/wayang-spark/code/test/java/org/apache/wayang/spark)

The idea is to create a similar operators test of platform-spark but in platform-flink

Start in the platform-flink code linked in the issue, then inspect the operator tests under platform-spark at test/java/org/apache/wayang/spark. Use the Spark operator tests as the reference for creating equivalent Flink coverage. Done means platform-flink has operator tests in addition to its existing integration test.

Written by the indexing model from the issue text.