[perl #131479] [ASYNC] Proc::Async.kill doesn't seem to work when more than 1 proc is involved

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# New Ticket Created by  Zoffix Znet 
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This appears to hang more often than not, despite printing "Killing" messages.

perl6 -e 'await ^2 .map: {start { with Proc::Async.new: $*EXECUTABLE, "-e", 
"sleep" -> $p { Promise.in(2).then: {say "Killing"; $p.kill: SIGTERM}; await 
$p.start } }}'

However, if we reduce it to just 1 Proc, then it appears to work without 
issues, killing the spawned Proc every time:

perl6 -e 'await ^1 .map: {start { with Proc::Async.new: $*EXECUTABLE, "-e", 
"sleep" -> $p { Promise.in(2).then: {say "Killing"; $p.kill: SIGTERM}; await 
$p.start } }}'


This looks to be the code handling the kill op, but I don't know if it looks 
wrong or not:

https://github.com/MoarVM/MoarVM/blob/dff6a4198f44d0b2793863cc56cc41679c36677f/src/io/procops.c#L1072-L1085

void MVM_proc_kill_async(MVMThreadContext *tc, MVMObject *handle_obj, MVMint64 
signal) {
    /* Ensure it's a handle for a process. */
    if (REPR(handle_obj)->ID == MVM_REPR_ID_MVMOSHandle) {
        MVMOSHandle *handle = (MVMOSHandle *)handle_obj;
        if (handle->body.ops == &proc_op_table) {
            /* It's fine; send the kill by cancelling the task. */
            MVMIOAsyncProcessData *data = (MVMIOAsyncProcessData 
*)handle->body.data;
            data->signal = signal;
            MVM_io_eventloop_cancel_work(tc, data->async_task, NULL, NULL);
            return;
        }
    }
    MVM_exception_throw_adhoc(tc, "killprocasync requires a process handle");
}