Adding fix to make the test_runner functions pub

blog/content/edition-2/posts/04-testing/index.md

@@ -73,7 +73,7 @@ To implement a custom test framework for our kernel, we add the following to our
 #![test_runner(crate::test_runner)]
 
 #[cfg(test)]
-fn test_runner(tests: &[&dyn Fn()]) {
+pub fn test_runner(tests: &[&dyn Fn()]) {
     println!("Running {} tests", tests.len());
     for test in tests {
         test();