client.drwrap-test-detach failing flakily on Windows: timeout

Split from #4058
This is happening on GA Server2016 as well on both 32-bit and 64-bit (xref #4131).
Pasting from https://github.com/DynamoRIO/dynamorio/issues/4058#issuecomment-607571875
https://ci.appveyor.com/project/DynamoRIO/dynamorio/builds/31894097
```
[00:10:16] Start 208: code_api|client.drwrap-test-detach
[00:10:16] 208: Test command: C:\projects\dynamorio\build\build_debug-internal-32\bin32\drrun.exe "-s" "90" "-quiet" "-debug" "-use_dll" "C:/projects/dynamorio/build/build_debug-internal-32/lib32/debug/dynamorio.dll" "-exit0" "-noinject" "-stderr_mask" "0xC" "-msgbox_mask" "0" "-dumpcore_mask" "0x7d" "-staged" "-code_api" "--" "C:/projects/dynamorio/build/build_debug-internal-32/suite/tests/bin/client.drwrap-test-detach.exe"
[00:10:16] 208: Test timeout computed to be: 600
[00:10:16] 208: in dr_client_main
[00:12:06] 228/228 Test #208: code_api|client.drwrap-test-detach ...........................***Failed Required regular expression not found. Regex=[^in dr_client_main
[00:12:06] ?
[00:12:06] client done
[00:12:06] ?
[00:12:06] app done
[00:12:06] ?
[00:12:06] $
[00:12:06] ] 90.02 sec
```

The GA symptoms are the same: a 90s timeout after printing `dr_client_main`.