Mention test_main and blog_os::init in later posts

blog/content/second-edition/posts/05-cpu-exceptions/index.md

@@ -399,6 +399,10 @@ pub extern "C" fn _start() -> ! {
     // invoke a breakpoint exception
     x86_64::instructions::interrupts::int3(); // new
 
+    // as before
+    #[cfg(test)]
+    test_main();
+
     println!("It did not crash!");
     loop {}
 }