Enrich testing doc with more unit test examples using AbstractStreamOperator
It seems that there are more and more users from user mailing list ask how
to do unit test with Flink
features like states or timer. And the community usually tends to suggest
`AbstractStreamOperator` and provide an example from Flink github repo.
Here I sort out some
examples and write them down in the testing documentation . And I would
link to contribute back
to the Flink.
The reason why I ask it first in dev mailing list is that
`AbstractStreamOperator` is an internal API and
could be changed at any time. I'm not sure if it is worth to provide these
examples on testing
document, so I want to collect some feedbacks before I go to open a JIRA
If this is feasible and valuable, then I will open the corresponding JIRA
ticket and we can discuss
more details of what examples are good to have in the document or how to
structure the content.
I would really appreciate any feedback from you. Thanks in advance.