diff: add a test case for failed -s option

usr.bin/diff/tests/diff_test.sh

@@ -15,6 +15,7 @@ atf_test_case Nflag
 atf_test_case tabsize
 atf_test_case conflicting_format
 atf_test_case label
+atf_test_case report_identical
 
 simple_body()
 {
@@ -226,6 +227,15 @@ label_body()
 		-s exit:1 diff --label hello --label world `which diff` `which ls`
 }
 
+report_identical_body()
+{
+	printf "\tA\n" > A
+	printf "\tB\n" > B
+	chmod -r B
+	atf_check -s exit:2 -e inline:"diff: B: Permission denied\n" \
+		-o empty diff -s A B
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case simple
@@ -243,4 +253,5 @@ atf_init_test_cases()
 	atf_add_test_case tabsize
 	atf_add_test_case conflicting_format
 	atf_add_test_case label
+	atf_add_test_case report_identical
 }