Mention test_main and blog_os::init in later posts

blog/content/second-edition/posts/06-double-faults/index.md

@@ -42,6 +42,10 @@ pub extern "C" fn _start() -> ! {
         *(0xdeadbeef as *mut u64) = 42;
     };
 
+    // as before
+    #[cfg(test)]
+    test_main();
+
     println!("It did not crash!");
     loop {}
 }
@@ -171,8 +175,7 @@ pub extern "C" fn _start() -> ! {
     // trigger a stack overflow
     stack_overflow();
 
-    println!("It did not crash!");
-    loop {}
+    […] // test_main(), println(…), and loop {}
 }
 ```