HANG timeout on win32.fiber-rac

Failed on a master merge:
https://github.com/DynamoRIO/dynamorio/runs/2221758433?check_suite_focus=true
```
test 27
Start 27: code_api|win32.fiber-rac

27: Test command: D:\a\dynamorio\dynamorio\build_debug-internal-64\bin64\drrun.exe "-s" "90" "-quiet" "-debug" "-use_dll" "D:/a/dynamorio/dynamorio/build_debug-internal-64/lib64/debug/dynamorio.dll" "-exit0" "-stderr_mask" "0xC" "-msgbox_mask" "0" "-dumpcore_mask" "0x7d" "-staged" "-code_api" "--" "D:/a/dynamorio/dynamorio/build_debug-internal-64/suite/tests/bin/win32.fiber-rac.exe"
27: Test timeout computed to be: 600
69/257 Test #27: code_api|win32.fiber-rac .....................................***Failed Required regular expression not found. Regex=[^in main thread
?
main thread converted to fiber
?
creating worker fiber 0
?
switching to worker fiber first time
?
in worker fiber
?
back to main
?
switching to worker fiber second time
?
in worker fiber again
?
deleting worker fiber 0
?
creating worker fiber 1
?
switching to worker fiber first time
?
in worker fiber
?
back to main
?
switching to worker fiber second time
?
in worker fiber again
?
deleting worker fiber 1
?
all done
?
$
] 90.03 sec
```