man.t: strip ansi escape codes

On FreeBSD the `ack --man` output read by man.t contains ANSI escape
codes, which breaks the test.  Update strip_special_chars to also
strip ANSI escape codes, fixing the test on FreeBSD.
Merge pull request #409 from frasertweedale/fix/man.t-strip-ansi

man.t: strip ansi escape codes