Listing 12-15 incorrectly marked as does_not_compile

[NeatNit]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • chapter 12 does not compile
  • listing 12-15
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • /src/ch12-04-testing-the-librarys-functionality.md

URL to the section(s) of the book with this problem:
https://doc.rust-lang.org/book/ch12-04-testing-the-librarys-functionality.html#listing-12-15

Description of the problem:
Listing 12-15 in Chapter 12.4 has the "does not compile" icon, but in reality it compiles just fine, and the test fails (as expected) with cargo test.

book/src/ch12-04-testing-the-librarys-functionality.md

Line 40 in 555182e

```rust,ignore,does_not_compile

Suggested fix:
Remove the does_not_compile flag/annotation.
Optionally replace with "panics" (if it counts).