rustdoc: add the ability to build a doctest executable without running it

[jsgf]

For flexibility of integration with other testing frameworks/infrastructure, it would be nice if rustdoc had a mechanism to build doctest executables without running them, so that they can be later run within a separate test coordination framework.

[fmease]

I'm inclined to close this in favor of #56925, the tracking issue for the experimental --persist-doctests. However, I might be missing some subtleties here.

[griff]

I'm inclined to close this in favor of #56925, the tracking issue for the experimental --persist-doctests. However, I might be missing some subtleties here.

Well --persist-doctests still runs the tests. And if you want to build the tests in one environment (where you don't have available the programs, test data and/or permissions needed to actually run the tests) and run the tests in a separate environment you are still out of luck with just --persist-doctests.