Python TestPipeline example code does not execute

The example python code in the last section of  [https://beam.apache.org/documentation/pipelines/test-your-pipeline/](https://beam.apache.org/documentation/pipelines/test-your-pipeline/) does not work. TestPipeline is not in beam, it is in beam.pipeline_test and the "with" takes an object, not a class (ie.  there needs to be a pair of brackets).  I suspect there are other issues, but those are the ones which I have noticed.

Imported from Jira [BEAM-10767](https://issues.apache.org/jira/browse/BEAM-10767). Original Jira may contain additional context.
Reported by: ensslen.