[FLINK-7554][Tests] Add a testing RuntimeContext to test utilities

[Jiayi-Liao]

What is the purpose of the change

This pull request adds a TestingRuntimeContext to help developers test their own user-defined functions like CoprocessFunction instead of testing the whole pipeline.

Brief change log

• TestingRuntimeContext can be directly used in unit testing of both dataset functions and datastream functions, including the support of broadcastVariable, state, accumulator and so on.
• Documents are written in udf_test, including examples of Java and Scala.

Verifying this change

• This change has already been verified in new unit testing files in test directory, org.apache.flink.streaming.api.functions.test.TestingRuntimeContextTest and org.apache.flink.streaming.api.functions.test.TestingRuntimeContextTest.

Documentation

• This pull request is a new feature.
• Related documents are written in docs directory.