tests/cksum: test when the --length argument is 0 or >512

tests/by-util/test_cksum.rs

@@ -262,3 +262,27 @@ fn test_length() {
         .succeeds()
         .stdout_is_fixture("supported_length.expected");
 }
+
+#[test]
+fn test_length_greater_than_512() {
+    new_ucmd!()
+        .arg("--length=1024")
+        .arg("--algorithm=blake2b")
+        .arg("lorem_ipsum.txt")
+        .arg("alice_in_wonderland.txt")
+        .fails()
+        .no_stdout()
+        .stderr_is_fixture("length_larger_than_512.expected");
+}
+
+#[test]
+fn test_length_is_zero() {
+    new_ucmd!()
+        .arg("--length=0")
+        .arg("--algorithm=blake2b")
+        .arg("lorem_ipsum.txt")
+        .arg("alice_in_wonderland.txt")
+        .succeeds()
+        .no_stderr()
+        .stdout_is_fixture("length_is_zero.expected");
+}

tests/fixtures/cksum/length_is_zero.expected

@@ -0,0 +1,2 @@
+BLAKE2b (lorem_ipsum.txt) = 0e97a09189e560c3789c0bff1f020166861ef857d1fbfe4574de1842e3c06cabb9575e4af6309a166158c2b408d3c038c1b49d828b35158142cdc0396d1195c3
+BLAKE2b (alice_in_wonderland.txt) = 91b8b0f0868e905ad18b8ac35e4a1dacd289857b19258ab5d1e071761af758b0134ec152d4f011fe1825ca889c80c2e072ca70eb50548c25fc49a98937515af4

tests/fixtures/cksum/length_larger_than_512.expected

@@ -0,0 +1,2 @@
+cksum: invalid length: ‘1024’
+cksum: maximum digest length for ‘BLAKE2b’ is 512 bits