Auto merge of #137899 - notriddle:merged-doctests-stable, r=fmease,GuillaumeGomez

doctests: fix merging on stable

Fixes #137898

The generated multi-test harness relies on nightly-only APIs, so the only way to run it on stable is to enable them.

To prevent the executing test case from getting at any of the stuff that the harness uses, they're built as two separate crates. The test bundle isn't built with RUSTC_BOOTSTRAP, while the runner harness is.