[Go-SDK] Add light example output helpers for documentation

Go has facilities to have "live" examples that run as part of the test suite, and are incorporated directly into the package documentation. These can also sanity check that text sent to STDOUT matches an expected set of lines.

This task has two desired components:
* A way to run the direct runner with significantly fewer lines of debugging output.
* A light type checker that outputs PCollection types. 

These would permit more expressive examples, and turn examples into tests. This also avoids oddities such as unused variables that need to be assigned to  _ to satisfy the Go Compiler. ("_ = foo_unused").

Imported from Jira [BEAM-4156](https://issues.apache.org/jira/browse/BEAM-4156). Original Jira may contain additional context.
Reported by: lostluck.