extra/runner/report: Avoid `sort` errors on illegal byte sequences

On a particular testrun (with truncated/corrupt logfile?) I got this
error from `sort`:

sort: string comparison failed: Illegal byte sequence
sort: Set LC_ALL='C' to work around the problem.
sort: The strings compared were
  `[2013-09-16 09:32:42.708681 +0100]  4\335\000\000' and
  `[2013-09-16 09:32:42.709526 +0100]  Traceback (most recent call last):'.