Stop real threadpool for test DB pool before replacing it with threadless test implementation (#20272)

changelog.d/20272.bugfix

@@ -0,0 +1 @@
+Prevent `trial` tests from hanging when running with Twisted 26.4.0 against Postgres.

tests/server.py

@@ -889,9 +889,15 @@ def make_fake_db_pool(
 
     pool.runWithConnection = runWithConnection  # type: ignore[method-assign]
     pool.runInteraction = runInteraction  # type: ignore[assignment]
-    # Replace the thread pool with a threadless 'thread' pool
+
+    # First, stop the original thread pool.
+    pool.threadpool.stop()
+    # Then, replace it with a threadless 'thread' pool
     pool.threadpool = ThreadPool(reactor)
+
+    # Start it up.
     pool.running = True
+
     return pool