Implement simple property testing

[tokenrove]

Ever since I rewrote the tests with silly Unicode characters, I wanted to have placeholders like σ for an arbitrarily quoted string, φ for a random file name, Γ for a globbable set of files and γ for the glob that matches, et cetera, to provide a modicum of property testing. To be effective, this should have shrinking, as well as a way to get the normative definitions of these variables (currently typically foo and such).