Add snapshot test for Center/Middle.

tests/snapshot_tests/snapshot_apps/alignment_containers.py

@@ -0,0 +1,29 @@
+"""
+App to test alignment containers.
+"""
+
+from textual.app import App, ComposeResult
+from textual.containers import Center, Middle
+from textual.widgets import Button
+
+
+class AlignContainersApp(App[None]):
+    CSS = """
+    Center {
+        tint: $primary 10%;
+    }
+    Middle {
+        tint: $secondary 10%;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        with Center():
+            yield Button.success("center")
+        with Middle():
+            yield Button.error("middle")
+
+
+app = AlignContainersApp()
+if __name__ == "__main__":
+    app.run()