run/logging-manual: Add e2e test coverage

[grayside]

This sample does not have end-to-end test coverage. This coverage should be added.

The existing unit test checks we write the expected log. The essential benefit of an end-to-end test is confirmation that Cloud Logging is successfully parsing the data structure. There is a node.js implementation of this test in nodejs-docs-samples:/run/logging-manual.

This issue is blocked on finding a solution for testing log details in #696.

[tbpg]

@grayside what is the next step here? #696 was resolved.

[grayside]

That's great news! The next step is to create an e2e test that publishes a message to a Pub/Sub topic to which this sample is subscribed, then check the expected log output.

[grayside]

Logging samples are being re-thought in the context of structure logging. Will leave sorting out testing to that effort.