fix markdown doc test code hiding to match librustdoc (#751)

https://github.com/rust-lang/rust/blob/0ecbd0605770f45c9151715e66ba2b3cae367fcb/src/librustdoc/html/markdown.rs#L170
https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html#hiding-portions-of-the-example

content/tokio/topics/testing.md

@@ -166,7 +166,7 @@ be used as a mock:
 #         line.clear();
 #     }
 # }
-
+#
 #[tokio::test]
 async fn client_handler_replies_politely() {
     let reader = tokio_test::io::Builder::new()

lib/markdown.js

@@ -18,7 +18,19 @@ const remarkCodeRemoveSomeLines = () => {
       if (node.lang && langs.includes(node.lang)) {
         node.value = node.value
           .split("\n")
-          .filter((line) => !line.startsWith("# "))
+          .map((line) => {
+            let trimmed = line.trim();
+            if (trimmed.startsWith("##")) {
+              return line.replace("##", "#");
+            } else if (trimmed.startsWith("# ")) {
+              return null;
+            } else if (trimmed === "#") {
+              return null;
+            } else {
+              return line;
+            }
+          })
+          .filter((line) => line !== null)
           .join("\n");
       }
     });