Rollup merge of #125753 - Zalathar:procres, r=jieyouxu

compiletest: Unify `cmd2procres` with `run_command_to_procres`

Historical context: I originally added `run_command_to_procres` in #112300 and #114843, because I didn't like the overly-specific failure message in `cmd2procres`, but at the time I didn't feel confident enough to change the existing code, so I just added my own similar code.

Now I'm going back to remove this redundancy by eliminating `cmd2procress`, and adjusting all callers to use a slightly-tweaked `run_command_to_procres` instead.

Author: matthiaskrgr