Currently assess gathers data by building projects in test mode. This is required for most of assess, but it is possible to run --codegen-only and just emit (statically reachable) unsupported features data.

I ran some experiments in #2029 to see what unsupported features are reported if we just built the package normally. We could add a flag to do this, instead of as a hack.

As a hack, the following two changes are made:

1. Patch cargokani_assess_main to no longer set tests = true
2. Patch cargo_build to force the reachability mode to ReachabilityMode::AllPubFns

We could add this is a proper feature by:

1. Add a new flag (or other mechanism) to cargo kani to change the reachability mode
2. Add a new flag (--no-tests?) to assess, to indicate this mode should be used.