`ThreadPool` problem

[drslebedev]

There appears to be a potential problem with the ThreadPool where not all threads are properly terminated or stopped before a new test begins. This issue was observed in the qa_Scheduler test. As a temporary workaround, a separate ThreadPool object is created for each test, as implemented in this commit:https://github.com/fair-acc/gnuradio4/commit/eea05ca8852f2f34a8f9fffc730cc8eb0db20dce.

This workaround also helped resolve a deadlock issue in the qa_Scheduler "blocking scheduler" test for the Emscripten Debug configuration.

The underlying problem needs further investigation. If you run the qa_Scheduler test with THREAD_SANITIZER=ON, you can see more details about the issue.