fix last test failure in buster-dev-tools/runtests

We were comparing a string with a buffer. That's due to an [API change in fs.readFileSync](http://nodejs.org/api/fs.html#fs_fs_readfilesync_filename_encoding), the key thing in its doc being
> If encoding is specified then this function returns a string. Otherwise it returns a buffer.

See also: [thread on dev mailing list](https://groups.google.com/forum/#!topic/busterjs-dev/KmM_3vEOHvs)