Add tests for Markdown broken links

src/scanners/markdown.rs

@@ -60,6 +60,15 @@ mod tests {
 ![Look, an image!](https://imgur.com/gallery/f28OkrB)
 
 [nowhere]: https://dev.null/
+
+- [x] Comments
+- [ ] Issues
+
+```sql
+ALTER FOREIGN TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
+    action [, ... ]
+```
+
         "#;
         let should_be = vec![
             (String::from("https://example.com"), Span::new(17, 44)),
@@ -75,4 +84,40 @@ mod tests {
 
         assert_eq!(got, should_be);
     }
+
+    #[test]
+    fn detect_broken_links_in_markdown() {
+        let src = r#"
+# Some Heading
+
+[this](https://example.com) is a link [to nowhere][nowhere]. But
+[this](../README.md) points somewhere on disk.
+
+![Look, an image!](https://imgur.com/gallery/f28OkrB)
+
+[nowhere]: https://dev.null/
+
+- [x] Comments
+- [ ] Issues
+
+```sql
+ALTER FOREIGN TABLE [ IF EXISTS ] [ ONLY ] name [ * ]
+    action [, ... ]
+```
+
+        "#;
+        let should_be = vec![
+            (String::from("https://example.com"), Span::new(17, 44)),
+            (String::from("https://dev.null/"), Span::new(55, 76)),
+            (String::from("../README.md"), Span::new(82, 102)),
+            (
+                String::from("https://imgur.com/gallery/f28OkrB"),
+                Span::new(130, 183),
+            ),
+        ];
+
+        let got: Vec<_> = markdown_with_broken_link_callback(src, &|a, b| Some((a.to_string(), b.to_string()))).collect();
+
+        assert_eq!(got, should_be);
+    }
 }