2024 doctests don't show stdout when failing

[GuillaumeGomez]

Can be seen with:

//! ```
//! println!("######## from a DOC TEST ########");
//! assert_eq!("doc", "test");
//! ```

Before 2024:

running 1 test
test src/lib.rs - (line 1) ... FAILED

failures:

---- src/lib.rs - (line 1) stdout ----
Test executable failed (exit status: 101).

stdout:
######## from a DOC TEST ########

stderr:

thread 'main' panicked at src/lib.rs:4:1:
assertion `left == right` failed
  left: "doc"
 right: "test"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

With 2024:

running 1 test
test src/lib.rs - (line 1) ... FAILED

failures:

---- src/lib.rs - (line 1) stdout ----

thread 'main' panicked at /tmp/rustdoctestMXBeHp/doctest_bundle_2024.rs:7:1:
assertion `left == right` failed
  left: "doc"
 right: "test"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Initially reported here.

cc @slluk

[GuillaumeGomez]

Fixed in #140291.