"An Example Test for a Composite Transform" in Python is broken

The example test code does not run [1]:
- Python reports a Syntax error "SyntaxError: unexpected EOF while parsing"
- Missing imports: "import unittest" and "import apache_beam as beam"
- After fixing the above issues, I get: "AttributeError: module 'apache_beam' has no attribute 'TestPipeline'". It looks like this import is just wrong. I tracked it down via code search in "apache_beam.testing.test_pipeline"

[1] https://beam.apache.org/documentation/pipelines/test-your-pipeline/#an-example-test-for-a-composite-transform

Imported from Jira [BEAM-12236](https://issues.apache.org/jira/browse/BEAM-12236). Original Jira may contain additional context.
Reported by: shoyer.