Replace tarball of test files with actual directory structure

[NicMcPhee]

We currently give them summarize_tree/test_data/test_data.tgz which they need to expand to get the directory structures fewer_files and loads_o_files expected by the BATS tests; if they don't, the tests fail in a somewhat confusing way.

We could open up that tar file and commit it as part of the repository they download so they can skip this step and focus on the C programming.

We should probably do a little experiment and make sure that won't make it a lot slower to clone the project, but assuming it doesn't this would probably be a good idea.

[NicMcPhee]

Having looked this over again after a few years, I have mixed feelings. That tar file has over 11K files in it, and I do suspect that if we were to expand it in GitHub that would certainly create some meaningful overhead that we arguably don't need.

[NicMcPhee]

Alternatively we could fancy up the bats tests so they see if the directory fewer_files exists, and if it doesn't it could expand the tarball.