Move the TreeNode label tests into a better-named file

There's going to be a whole bunch of tests relating to the Tree and TreeNode
coming, let's make sure this ends up being fairly granular.

(side thought: it might be a good time soon to revisit all the tests for
Textual and try and wrangle them into some tidy structure)

tests/tree/test_tree_node_label.py

@@ -0,0 +1,17 @@
+from textual.widgets import Tree, TreeNode
+from rich.text import Text
+
+def test_tree_node_label() -> None:
+    """It should be possible to modify a TreeNode's label."""
+    node = TreeNode(Tree[None]("Xenomorph Lifecycle"), None, 0, "Facehugger")
+    assert node.label == Text("Facehugger")
+    node.label = "Chestbuster"
+    assert node.label == Text("Chestbuster")
+
+def test_tree_node_label_via_tree() -> None:
+    """It should be possible to modify a TreeNode's label when created via a Tree."""
+    tree = Tree[None]("Xenomorph Lifecycle")
+    node = tree.root.add("Facehugger")
+    assert node.label == Text("Facehugger")
+    node.label = "Chestbuster"
+    assert node.label == Text("Chestbuster")