Create test framework in t/ and t/TestKit.pm for testing expressions

We need to be able to test operations on Git repos. [Minilla](https://github.com/tokuhirom/Minilla) might be good as a source of ideas (but not code), since it has the same need.

- [ ] Make a hierarchy under t/data that has various directories, subdirectories, ...
- [ ] ? Add a submodule for purposes of author testing?
- [ ] Add functions to t/TestKit.pm to make it easier to run tests against the repo when checked out, and to skip those tests if we're running `make test` in an extracted tarball (as opposed to a clone).
- [ ] Add functional tests! E.g., does `-print` actually print the filenames we expect?