Instead of redirecting output to pipes and forking, redirect after
forking to avoid having to carefully unredirect before logging
anything.

igt@tools_test@sysfs_l3_parity had a racy condition where it prints
the output of intel_l3_parity prepended by [cmd], but that ended up
being printed again prepended by [cmd] because output was redirected,
causing outputs to appear multiple times. This patch fixes that.

CC: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>