Remove some old cfg(not(test)) attributes

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

@@ -390,7 +390,6 @@ Now we can update the `_start` function of our `main.rs` to call `init` and then
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     println!("Hello World{}", "!");

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

@@ -31,7 +31,6 @@ Let's provoke a double fault by triggering an exception for that we didn't defin
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     println!("Hello World{}", "!");
@@ -159,7 +158,6 @@ Let's try it ourselves! We can easily provoke a kernel stack overflow by calling
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle] // don't mangle the name of this function
 pub extern "C" fn _start() -> ! {
     println!("Hello World{}", "!");

blog/content/second-edition/posts/07-hardware-interrupts/index.md

@@ -309,7 +309,6 @@ We can easily provoke such a deadlock in our kernel by printing something in the
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     […]

blog/content/second-edition/posts/08-paging-introduction/index.md

@@ -303,7 +303,6 @@ Now we can try to access some memory outside our kernel:
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     use blog_os::interrupts::PICS;
@@ -353,7 +352,6 @@ Let's try to take a look at the page tables that our kernel runs on:
 ```rust
 // in src/main.rs
 
-#[cfg(not(test))]
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
     […] // initialize GDT, IDT, PICS