gh-132969: Fix error/hang when shutdown(wait=False) and task exited abnormally (GH-133222)

When shutdown is called with wait=False, the executor thread keeps running
even after the ProcessPoolExecutor's state is reset. The executor then tries
to replenish the worker processes pool resulting in an error and a potential hang
when it comes across a worker that has died. Fixed the issue by having
_adjust_process_count() return without doing anything if the ProcessPoolExecutor's
state has been reset.

Added unit tests to validate two scenarios:
max_workers < num_tasks (exception)
max_workers > num_tasks (exception + hang)

Lib/concurrent/futures/process.py

@@ -755,6 +755,11 @@ class ProcessPoolExecutor(_base.Executor):
                 self._executor_manager_thread_wakeup
 
     def _adjust_process_count(self):
+        # gh-132969: avoid error when state is reset and executor is still running,
+        # which will happen when shutdown(wait=False) is called.
+        if self._processes is None:
+            return
+
         # if there's an idle process, we don't need to spawn a new one.
         if self._idle_worker_semaphore.acquire(blocking=False):
             return