tests: Work around a Textual deadlock

There's a deadlock on shutdown when the pilot exits before the message
pump has quiesced, but it doesn't occur when we wait for the message
pump to become idle.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

tests/unit/test_tui_reporter.py

@@ -462,8 +462,9 @@ def on_snapshot_fetched(self, message):
 
     # WHEN
     async def run_test():
-        async with app.run_test():
+        async with app.run_test() as pilot:
             await all_messages_received.wait()
+            await pilot.pause()
 
     async_run(run_test())