test: remove util from common

util is loaded just for one use of util.format(). Replace it with a
string template. While we're at it, delete nearby lengthy comment
justifying use of fs.readFileSync().

PR-URL: https://github.com/nodejs/node/pull/3324


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed By: Evan Lucas <evanlucas@me.com>