diff: add a test case about the non regular file support

(cherry picked from commit c440e7870a)

usr.bin/diff/tests/diff_test.sh

@@ -16,6 +16,7 @@ atf_test_case tabsize
 atf_test_case conflicting_format
 atf_test_case label
 atf_test_case report_identical
+atf_test_case non_regular_file
 
 simple_body()
 {
@@ -236,6 +237,19 @@ report_identical_body()
 		-o empty diff -s A B
 }
 
+non_regular_file_body()
+{
+	printf "\tA\n" > A
+	mkfifo B
+	printf "\tA\n" > B &
+
+	atf_check diff A B
+	printf "\tB\n" > B &
+	atf_check -s exit:1 \
+		-o inline:"--- A\n+++ B\n@@ -1 +1 @@\n-\tA\n+\tB\n" \
+		diff --label A --label B -u A B
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case simple
@@ -254,4 +268,5 @@ atf_init_test_cases()
 	atf_add_test_case conflicting_format
 	atf_add_test_case label
 	atf_add_test_case report_identical
+	atf_add_test_case non_regular_file
 }