Rollup merge of #146574 - Zalathar:capture, r=jieyouxu

compiletest: Enable new-output-capture by default

The new output-capture implementation was added in rust-lang/rust#146119, but was disabled by default and required opt-in.

Since then, I haven't encountered any problems in my own testing/usage, and I haven't heard any problem reports from other contributors who might have opted in.

It's unlikely that more opt-in testing will help, so the next step is to enable new-output-capture by default and see if anyone complains. (Hopefully nobody!)

If needed, the new default can be overridden (for now) by setting environment variable `COMPILETEST_NEW_OUTPUT_CAPTURE=off`. Please file an issue (or let me know) if anyone finds a reason to do this.

r? jieyouxu

Author: Zalathar