client.detach_test failing: kill cannot find target process

The Linux client.detach_test was just added in PR #6513 and it is failing (non-deterministically) on x86-64.

https://github.com/DynamoRIO/dynamorio/actions/runs/7381905578/job/20080993968?pr=6531
```
Test #185: code_api|client.detach_test ......................................***Failed Required regular expression not found. Regex=[^starting
thank you for testing detach
thread init
detach
done
$
] 0.26 sec
CMake Error at /home/runner/work/dynamorio/dynamorio/suite/tests/runall.cmake:130 (message):
*** kill failed (1): kill: (24600): No such process
***
Call Stack (most recent call first):
/home/runner/work/dynamorio/dynamorio/suite/tests/runall.cmake:317 (kill_background_process)
```