Update Python SDK example tests to use assert_that

[kennknowles]

Most of our examples use assert_that to test examples:

https://github.com/apache/incubator-beam/blob/python-sdk/sdks/python/apache_beam/examples/complete/autocomplete_test.py#L38

Some of our examples use this pattern:

1. Create a path(s)
2. Construct fake command line arguments using these paths
3. Construct an argparse object to parse these flags
4. Do the (often trivial logic)
5. Write to a file
6. Manually open and read the file
7. Compare results.

https://github.com/apache/incubator-beam/blob/python-sdk/sdks/python/apache_beam/examples/cookbook/multiple_output_pardo_test.py

As well as being cumbersome, this obscures the core of what is being illustrated and tested. As many as possible tests should be updated to use assert_that.

Imported from Jira BEAM-520. Original Jira may contain additional context.
Reported by: altay.

[liferoad]

https://github.com/apache/beam/blob/master/sdks/python/apache_beam/examples/cookbook/multiple_output_pardo_test.py

Probably keeping the current code is good for now. We can always clean them up one by one when these needs to be improved. Close this for now.