Auto merge of #146076 - Kobzol:bootstrap-compiletest, r=jieyouxu

Consolidate staging for compiletest steps in bootstrap

This PR finishes the initial pass of refactorings that fixed up staging of various bootstrap steps after the stage0 redesign. Now the *real* refactorings can begin =D

It fixes the unnecessary stage2 build of cargo for run-make tests (https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/run-make.20cargo.20staging/with/536662651), and also consolidates staging of messages printed by bootstrap for test steps (`Testing stageN...` etc.).

r? `@jieyouxu`

Author: bors