:rgrep : use atf-check to check the exit code/save the output of grep -r instead
of calling grep -r without it, and saving the output to a file

This ensures that any errors thrown via grep -r are caught, not lost, and uses
existing atf-sh idioms for saving files.

Tested with: bsdgrep, gnu grep (base, ports)
Sponsored by: Dell EMC Isilon