bsdgrep: revise test case which will soon become a failure

Work in progress (D10315) is going to make egrep_empty_invalid an
actually invalid regex, to be consistent with the equivalent BRE "{"
behavior, when using regex(3).

Any non-0 exit value is acceptable, depending on how the installed grep
interprets the expression. GNU grep interprets it as non-matching, and
in the future BSD grep will interpret it is an error.

Submitted by:	Kyle Evans <kevans91 at ksu.edu>
Reviewed by:	cem, ngie
Differential Revision:	https://reviews.freebsd.org/D10572`

contrib/netbsd-tests/usr.bin/grep/t_grep.sh

@@ -369,7 +369,7 @@ egrep_empty_invalid_head()
 }
 egrep_empty_invalid_body()
 {
-	atf_check -s exit:1 egrep '{' /dev/null
+	atf_check -e ignore -s not-exit:0 egrep '{' /dev/null
 }
 
 atf_test_case zerolen