Modify failed pjlib test to exclusive

[sauwming]

Re #4246 (CI: change sleep_test() and timestamp_test() to exclusive on Windows), the failure also happens on other platforms such as Mac, not just on Windows.

Example log from CI Mac test:

06:09:03.889 ------------ Logs for sleep_test [rc:-30]: ------------
05:57:37.097 ...2025-00-21 05:57:37.097
05:57:38.139 ...2025-00-21 05:57:38.139
05:57:39.229 ...2025-00-21 05:57:39.229
05:57:39.229 ..running sleep duration test
05:57:41.379 ...error: slept for 2150 ms instead of 2000 ms (outside 20 msec tolerance)
05:57:42.403 ...error: slept for 1024 ms instead of 1000 ms (outside 20 msec tolerance)
05:57:42.977 ...error: slept for 574 ms instead of 500 ms (outside 20 msec tolerance)
05:57:43.305 ...error: slept for 121 ms instead of 100 ms (outside 20 msec tolerance)
05:57:43.305 ...avg/max slippage: 54/150 ms