Ability to add files without testing them

[langner]

It's common to add a regression file to this repo, and then write a regression test in the main repo at the same time. I feel like this process could be better, since we often just say "this needs to go in first" on one PR. Usually that's the data file, since the file is needed in the regression test itself.

Perhaps the right thing to do is move regressionfiles.txt to the main repo and not error out on missing files (or if there are less than N), so that here we just add files. Or maybe we just get rid of this file altogether, and track things in a different, smoother process. There needs to be a way.