client.attach_test failing sometimes: kill cannot find the target process

Split from #5725. This still sometimes happens, here on x64 Linux:

https://github.com/DynamoRIO/dynamorio/actions/runs/6855254564/job/18639921367?pr=6445
```
183/476 Test #183: code_api|client.attach_test ......................................***Failed Required regular expression not found. Regex=[^starting
thank you for testing attach
thread init
done
$
] 0.25 sec
CMake Error at /home/runner/work/dynamorio/dynamorio/suite/tests/runall.cmake:124 (message):
*** kill failed (1): kill: (24386): No such process

***
```