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

[cxw42]

We need to be able to test operations on Git repos. 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?