Add simple test to prove the scan for in-buffer settings is still working.

src/parser/in_buffer_settings.rs

@@ -113,3 +113,33 @@ pub(crate) fn apply_in_buffer_settings<'g, 's, 'sf>(
 
     Ok(new_settings)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn scan_test() -> Result<(), Box<dyn std::error::Error>> {
+        let input = OrgSource::new(
+            r#"
+foo
+   #+archive: bar
+
+baz #+category: lorem
+
+#+label: ipsum
+
+#+todo: dolar
+cat
+"#,
+        );
+        let (remaining, settings) = scan_for_in_buffer_settings(input)?;
+        assert_eq!(Into::<&str>::into(remaining), "cat\n");
+        let keys: Vec<_> = settings.iter().map(|kw| kw.key).collect();
+        // category is skipped because it is not the first non-whitespace on the line.
+        //
+        // label is skipped because it is not an in-buffer setting.
+        assert_eq!(keys, vec!["archive", "todo"]);
+        Ok(())
+    }
+}