Add snapshot test for hot reloading.

tests/snapshot_tests/snapshot_apps/hot_reloading_app.py

@@ -0,0 +1,28 @@
+from pathlib import Path
+
+from textual.app import App, ComposeResult
+from textual.containers import Container
+from textual.widgets import Label
+
+
+CSS_PATH = (Path(__file__) / "../hot_reloading_app.css").resolve()
+
+CSS_PATH.write_text(
+    """
+Container {
+    align: center middle;
+}
+
+Label {
+    border: round $primary;
+    padding: 3;
+}
+"""
+)
+
+
+class HotReloadingApp(App[None]):
+    CSS_PATH = CSS_PATH
+
+    def compose(self) -> ComposeResult:
+        yield Container(Label("Hello, world!"))