Rollup merge of #136556 - amy-kwan:amy-kwan/update_wait-forked-but-failed-child.rs, r=joboet

[AIX] Update tests/ui/wait-forked-but-failed-child.rs to accomodate exiting and idle processes.

The `wait-forked-but-failed-child.rs` test expects to see an integer PPID in the
output of the command: `ps -A -o pid,ppid,args`.

However, on AIX, sometimes an integer PPID is not available when a process is
either exiting or idle, as documented in https://www.ibm.com/docs/en/aix/7.3?topic=p-ps-command.
In these situations, a `-` is instead shown in the PPID column of the `ps` output.

This PR updates the test to accommodate this behaviour on AIX by first filtering out the
lines of the `ps` output where a `-` is found in the `PPID` column.

Author: matthiaskrgr