Start unit tests for live key bindings plus inheriting

Up until now there doesn't seem to have been any unit tests aimed squarely
at setting up bindings, as part of a running application, which are only
about testing the bindings. As such there was no way of slotting in tests
for how inheritance of bindings works.

This starts that process with a view to testing how inheriting
likely *should* work.

See #1343 for some background to this.

tests/test_binding_inheritance.py

@@ -0,0 +1,77 @@
+import pytest
+
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+from textual.screen import Screen
+from textual.binding import Binding
+
+##############################################################################
+class NoBindings(App[None]):
+    """An app with zero bindings."""
+
+
+async def test_just_app_no_bindings() -> None:
+    """An app with no bindings should have no bindings, other than ctrl+c."""
+    async with NoBindings().run_test() as pilot:
+        assert list(pilot.app._bindings.keys.keys()) == ["ctrl+c"]
+
+
+##############################################################################
+class AlphaBinding(App[None]):
+    """An app with a simple alpha key binding."""
+
+    BINDINGS = [Binding("a", "a", "a")]
+
+
+async def test_just_app_alpha_binding() -> None:
+    """An app with a single binding should have just the one binding."""
+    async with AlphaBinding().run_test() as pilot:
+        assert sorted(pilot.app._bindings.keys.keys()) == sorted(["ctrl+c", "a"])
+
+
+##############################################################################
+class ScreenNoBindings(Screen):
+    """A screen with no added bindings."""
+
+
+class AppWithScreenNoBindings(App[None]):
+    """An app with no extra bindings but with a custom screen."""
+
+    SCREENS = {"main": ScreenNoBindings}
+
+    def on_mount(self) -> None:
+        self.push_screen("main")
+
+
+@pytest.mark.xfail(
+    reason="Screen is incorrectly starting with bindings for movement keys [issue#1343]"
+)
+async def test_app_screen_no_bindings() -> None:
+    """An screen with no bindings should have no bindings."""
+    async with AppWithScreenNoBindings().run_test() as pilot:
+        assert list(pilot.app.screen._bindings.keys.keys()) == []
+
+
+##############################################################################
+class ScreenWithBindings(Screen):
+    """A screen with a simple alpha key binding."""
+
+    BINDINGS = [Binding("a", "a", "a")]
+
+
+class AppWithScreenThatHasABinding(App[None]):
+    """An app with no extra bindings but with a custom screen with a binding."""
+
+    SCREENS = {"main": ScreenWithBindings}
+
+    def on_mount(self) -> None:
+        self.push_screen("main")
+
+
+@pytest.mark.xfail(
+    reason="Screen is incorrectly starting with bindings for movement keys [issue#1343]"
+)
+async def test_app_screen_with_bindings() -> None:
+    """An app with a screen and a binding should only have ctrl+c as a binding."""
+    async with AppWithScreenThatHasABinding().run_test() as pilot:
+        assert list(pilot.app.screen._bindings.keys.keys()) == ["a"]