Auto merge of #140288 - Zalathar:new-executor, r=jieyouxu

compiletest: Re-land using the new non-libtest executor by default

This PR re-lands #139998, which had the misfortune of triggering download-rustc in its CI jobs, so we didn't get proper test metrics for comparison with the old implementation. So that was PR was reverted in #140233, with the intention of re-landing it alongside a dummy compiler change to inhibit download-rustc.

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Original PR description for #139998:
>The new executor was implemented in #139660, but required a manual opt-in. This PR activates the new executor by default, but leaves the old libtest-based executor in place (temporarily) to make reverting easier if something unexpectedly goes horribly wrong.
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>Currently the new executor can be explicitly disabled by passing the `-N` flag to compiletest (e.g. `./x test ui -- -N`), but eventually that flag will be removed, alongside the removal of the libtest dependency. The flag is mostly there to make manual comparative testing easier if something does go wrong.
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>As before, there *should* be no user-visible difference between the old executor and the new executor.

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r? jieyouxu

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