Rollup merge of #146119 - Zalathar:capture, r=jieyouxu

compiletest: Implement an experimental `--new-output-capture` mode

Thanks to the efforts on rust-lang/rust#140192, compiletest no longer has an unstable dependency on libtest, but it still has an unstable dependency on `#![feature(internal_output_capture)]`. That makes building compiletest more complicated than for most other bootstrap tools.

This PR therefore adds opt-in support for an experimental compiletest mode that avoids the use of `internal_output_capture` APIs, and instead uses more mundane means to capture the output of individual test runners.

Each `TestCx` now contains `&dyn ConsoleOut` references for stdout and stderr. All print statements in `compiletests::runtest` have been replaced with `write!` or `writeln!` calls that explicitly write to one of those trait objects. The underlying implementation then forwards to `print!` or `eprint!` (for `--no-capture` or old-output-capture mode), or writes to a separate buffer (in new-output-capture mode).

---

Currently, new-output-capture is disabled by default. It can be explicitly enabled in one of two ways:

- When running `x test`, pass `--new-output-capture=on` as a *compiletest* argument (after `--`).
  - E.g. `x test ui -- --new-output-capture=on`.
  - The short form is `-Non` or `-Ny`.
- Set environment variable `COMPILETEST_NEW_OUTPUT_CAPTURE=on`.

After some amount of opt-in testing, new-output-capture will become the default (with a temporary opt-out). Eventually, old-output-capture and `#![feature(internal_output_capture)]` will be completely removed from compiletest.

r? jieyouxu

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