Where can I find a whole bunch of complicated PGN files? I'm interested to test that the atom syntax highlighter for pgn files -- https://atom.io/packages/language-pgn -- can handle the full range of possible files.  Is there a suite of difficult test files somewhere?  

Also related might be this question:

What are all of the possible tags for PGN?
As an author of some PGN-processing software, I would also be interested in having access to a general test suite but I am not aware of any, unfortunately, and all I can offer is some advice based on experience.

I would be wary of throwing a large database of games at an application in order to test it before you have developed reasonable confidence with its fundamentals. In the early stages, at least, I find it most effective to create a lot of small examples, each of which tests an individual feature. That way, errors are relatively easy to spot because they don't get lost in a mass of output. Once I have got rid of the basic errors I would then look at combining a small number of both related and unrelated features in individual tests to make sure that there is no unexpected interference between the associated functionality. Only late on would I switch to large datasets to make sure there are no issues with scaling up.

I also highly recommend re-running all the existing tests each time a new feature is added. I have been amazed how often I have unexpectedly broken something seemingly unrelated as the code has developed.

Apologies if these principles are already well known to you!