Rollup merge of #70662 - eddyb:compiletest-stdout-fix, r=Mark-Simulacrum

compiletest: don't use `std::io::stdout()`, as it bypasses `set_print`.

This PR undoes a change made during #69916, which became unnecessary during review but was left in by accident, and which isn't correct due to `libtest` using `std::io::set_print`, which overwrites the `println!` behavior but *not* `writeln!(std::io::stdout(), ...)`.

The effect of using `writeln!(std::io::stdout(), ...)` was that the diff output would show *while* running the tests, instead of at the end, when failing tests are listed.

r? @Mark-Simulacrum cc @oli-obk

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