Force a wee wait when testing the demo

This keeps randomly failing in Windows in CI; multiple subsequent runs gets
it going in the end, normally one further fail at a time. So let's throw a
wee wait on the end and see if that helps.

tests/snapshot_tests/test_snapshots.py

@@ -295,7 +295,7 @@ def test_demo(snap_compare):
     """Test the demo app (python -m textual)"""
     assert snap_compare(
         Path("../../src/textual/demo.py"),
-        press=["down", "down", "down"],
+        press=["down", "down", "down", "wait:250"],
         terminal_size=(100, 30),
     )