Rollup merge of #144805 - Zalathar:proc-res, r=jieyouxu

compiletest: Preliminary cleanup of `ProcRes` printing/unwinding

While experimenting with changes to how compiletest handles output capture, error reporting, and unwinding, I repeatedly ran in to difficulties with this core code for reporting test failures caused by a subprocess.

There should be no change in compiletest output.

r? jieyouxu

Author: samueltardieu