[llvm] [lit] Collect completed tests via callbacks, not wait() (PR #214386)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> This looks reasonable to me. Do we have numbers here? Also some feedback from @c-rhodes about how well this addresses the issues they saw, would be good to know.

https://github.com/llvm/llvm-project/pull/209076#issuecomment-5201237895

https://github.com/llvm/llvm-project/pull/214386