ZTS: Use POSIX-compatible space character class

FreeBSD recently integrated a change which causes \s in a regex to
throw an error instead of silently being misinterpreted as an s.

Change the regex in zpool_colors.ksh to use :space:.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10651