: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

usr.bin/grep/tests/grep_freebsd_test.sh

@@ -77,8 +77,7 @@ rgrep_head()
 }
 rgrep_body()
 {
-	grep -r --exclude="*.out" -e "test" "$(atf_get_srcdir)" > d_grep_r_implied.out
-
+	atf_check -o save:d_grep_r_implied.out grep -r --exclude="*.out" -e "test" "$(atf_get_srcdir)"
 	atf_check -o file:d_grep_r_implied.out rgrep --exclude="*.out" -e "test" "$(atf_get_srcdir)"
 }