i#6536: eliminate the instability of detachment on x86-64 platform

[onroadmuwl]

Print the done in detach_test.dll.c instead of infloop.c.
Fixes: #6536

[derekbruening]

I re-ran the x86-64 job and the detach test still fails:

185/486 Test #185: code_api|client.detach_test ......................................***Failed  Required regular expression not found. Regex=[^starting
thank you for testing detach
thread init
detach
done
$
]  0.27 sec
CMake Error at /home/runner/work/dynamorio/dynamorio/suite/tests/runall.cmake:130 (message):
  *** kill failed (1): kill: (27149): No such process
  ***

I also took this patch and ran it locally and it fails there too in the same way.

[abhinav92003]

I see more changes were made since Derek's last comment. When this PR is ready for a re-review, please use the Github UI to signal that (see list of reviewers on top-right side of the page).