diff: Use unprivileged_user with report_identical test

Approved by:	bapt
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D28466

(cherry picked from commit b67df8d7c2)

usr.bin/diff/tests/diff_test.sh

@@ -228,13 +228,18 @@ label_body()
 		-s exit:1 diff --label hello --label world `which diff` `which ls`
 }
 
+report_identical_head()
+{
+	atf_set "require.config" unprivileged_user
+}
 report_identical_body()
 {
+	UNPRIVILEGED_USER=$(atf_config_get unprivileged_user)
 	printf "\tA\n" > A
 	printf "\tB\n" > B
 	chmod -r B
 	atf_check -s exit:2 -e inline:"diff: B: Permission denied\n" \
-		-o empty diff -s A B
+		-o empty su -m "$UNPRIVILEGED_USER" -c 'diff -s A B'
 }
 
 non_regular_file_body()