test the binary

[amtoine]

Should we test the function format_file? we can write a file, format it and the test it. After that, we can delete the file.
What do you think? We can move that to another time too!

Originally posted by @AucaCoyan in #31 (comment)

as nufmt is both a library (nu-formatter) and a binary application (nufmt), i think we should test both at some point!

• we already started testing the library with format_string 👌
• we could take some inspiration from Nushell to test nufmt, you know with the nu! macro that runs commands in nu as if the tests were in the REPL