ignore code examples in tests

Author: nikomatsakis

src/rust-2021/IntoIterator-for-arrays.md

@@ -8,7 +8,7 @@ Until Rust 1.53, only *references* to arrays implement `IntoIterator`.
 This means you can iterate over `&[1, 2, 3]` and `&mut [1, 2, 3]`,
 but not over `[1, 2, 3]` directly.
 
-```rust
+```rust,ignore
 for &e in &[1, 2, 3] {} // Ok :)
 
 for e in [1, 2, 3] {} // Error :(

src/rust-2021/disjoint-capture-in-closures.md

@@ -15,7 +15,7 @@ When a field of the struct is already borrowed (mutably) or moved out of,
 the other fields can no longer be used in a closure,
 since that would capture the whole struct, which is no longer available.
 
-```rust
+```rust,ignore
 let a = SomeStruct::new();
 
 drop(a.x); // Move out of one field of the struct

src/rust-2021/panic-macro-consistency.md

@@ -9,15 +9,15 @@ The `panic!()` macro is one of Rust's most well known macros.
 However, it has [some subtle surprises](https://github.com/rust-lang/rfcs/blob/master/text/3007-panic-plan.md)
 that we can't just change due to backwards compatibility.
 
-```rust
+```rust,ignore
 panic!("{}", 1); // Ok, panics with the message "1"
 panic!("{}"); // Ok, panics with the message "{}"
 ```
 
 The `panic!()` macro only uses string formatting when it's invoked with more than one argument.
 When invoked with a single argument, it doesn't even look at that argument.
 
-```rust
+```rust,ignore
 let a = "{";
 println!(a); // Error: First argument must be a format string literal
 panic!(a); // Ok: The panic macro doesn't care