Improve coverage / error messages from parse_size PR

https://github.com/uutils/coreutils/pull/3084 (2a333ab391) had some
missing coverage and was merged before I had a chance to fix it.

This PR adds some coverage / improved error messages that were missing
from that previous PR.

tests/by-util/test_split.rs

@@ -342,6 +342,31 @@ fn test_split_invalid_bytes_size() {
     }
 }
 
+#[test]
+fn test_split_chunks_num_chunks_oversized_32() {
+    #[cfg(target_pointer_width = "32")]
+    {
+        let scene = TestScenario::new(util_name!());
+        let at = &scene.fixtures;
+        at.touch("file");
+        scene
+            .ucmd()
+            .args(&["--number", "5000000000", "file"])
+            .fails()
+            .code_is(1)
+            .stderr_only("split: Number of chunks too big");
+    }
+}
+
+#[test]
+fn test_split_stdin_num_chunks() {
+    new_ucmd!()
+        .args(&["--number=1"])
+        .fails()
+        .code_is(1)
+        .stderr_only("split: -: cannot determine file size");
+}
+
 fn file_read(at: &AtPath, filename: &str) -> String {
     let mut s = String::new();
     at.open(filename).read_to_string(&mut s).unwrap();