pythongh-109413: Fix libregrtest get_running() (pythonGH-127153)

Skip threads which are not running.
(cherry picked from commit 0cb2017)

Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/test/libregrtest/run_workers.py

@@ -451,7 +451,7 @@ def get_running(workers: list[WorkerThread]) -> str | None:
     running: list[str] = []
     for worker in workers:
         test_name = worker.test_name
-        if not test_name:
+        if test_name == _NOT_RUNNING:
             continue
         dt = time.monotonic() - worker.start_time
         if dt >= PROGRESS_MIN_TIME: