Add some unit tests for Tree messages/event handling

I'm about to work on #1400 and it seems like a good idea to put some tests
in place first to ensure nothing gets disturbed.

tests/tree/test_tree_messages.py

@@ -0,0 +1,54 @@
+from typing import Any
+from textual.app import App, ComposeResult
+from textual.widgets import Tree
+from textual.message import Message
+
+
+class TreeApp(App[None]):
+    """Test tree app."""
+
+    def __init__(self, *args: Any, **kwargs: Any) -> None:
+        super().__init__(*args, **kwargs)
+        self.messages: list[str] = []
+
+    def compose(self) -> ComposeResult:
+        """Compose the child widgets."""
+        yield Tree[None]("Root")
+
+    def on_mount(self) -> None:
+        """"""
+        self.query_one(Tree[None]).root.add("Child")
+        self.query_one(Tree[None]).focus()
+
+    def record(self, event: Message) -> None:
+        self.messages.append(event.__class__.__name__)
+
+    def on_tree_node_selected(self, event: Tree.NodeSelected[None]) -> None:
+        self.record(event)
+
+    def on_tree_node_expanded(self, event: Tree.NodeExpanded[None]) -> None:
+        self.record(event)
+
+    def on_tree_node_collapsed(self, event: Tree.NodeCollapsed[None]) -> None:
+        self.record(event)
+
+
+async def test_tree_node_selected_message() -> None:
+    """Selecting a node should result in a selected message being emitted."""
+    async with TreeApp().run_test() as pilot:
+        await pilot.press("enter")
+        assert pilot.app.messages[-1] == "NodeSelected"
+
+
+async def test_tree_node_expanded_message() -> None:
+    """Expanding a node should result in an expanded message being emitted."""
+    async with TreeApp().run_test() as pilot:
+        await pilot.press("enter")
+        assert pilot.app.messages[0] == "NodeExpanded"
+
+
+async def test_tree_node_collapsed_message() -> None:
+    """Collapsing a node should result in a collapsed message being emitted."""
+    async with TreeApp().run_test() as pilot:
+        await pilot.press("enter", "enter")
+        assert pilot.app.messages[-2] == "NodeCollapsed"