improving test failure message display

[imrekoszo]

Using pr-str when displaying the error message adds all sorts of goop like \ns to the output. As the message arg in an (is ...) form is usually a string, you probably don't need the pr-str around it. Sorry I didn't notice this earlier.

[honzabrecka]

You're absolutely right, thank you! I rebased your commit onto actual master, therefore closing manually.