* Fix hlint suggestions, update hlint.yaml
Most suggestions were redundant brackets. Some required
LambdaCase.
The .hlint.yaml file had a small typo, and didn't ignore camelCase
suggestions in certain modules.
There is very little pptx-specific in these tests, so we abstract out
the basic testing function so it can be used for docx as well. This
should allow us to catch some errors in the docx writer that slipped
by the roundtrip testing.