fix: test and cleanup learner report

[brianreichtcs]

If approved this PR makes some changes to the library to make Client::getLearnerReport() work as expected, and shores up issues with the unit tests that came up after the fix was made and verified.

What it doesn't do:

• Change the language of Learner Report to Enrollment Report. I will do that in a follow-up PR once this is approved and merged.
• Make too many other changes that weren't necessary to make this work. We know the team will benefit from a working query so we can immediately determine "what's possible" via the reporting API. There are likely code and test coverage improvements to be made but I wanted to take the path of least resistance to provide a proof of concept, so I can get back to doing other things and hand this project off to the team.

[tomegantcs]

We don't have a build process for this library to do replacements of the $version$ in file doc blocks; should we therefore drop the @version lines as well?

[brianreichtcs]

We don't have a build process for this library to do replacements of the $version$ in file doc blocks; should we therefore drop the @version lines as well?

I'll follow-up with another PR for that.